diff --git a/consensus/clique/clique.go b/consensus/clique/clique.go
index 0ff72e55c6..12199d7501 100644
--- a/consensus/clique/clique.go
+++ b/consensus/clique/clique.go
@@ -20,6 +20,7 @@ package clique
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"math/big"
 	"math/rand"
 	"sync"
@@ -166,19 +167,12 @@ func sigHash(header *types.Header) (hash common.Hash) {
 	return hash
 }
 
-// ecrecover extracts the Ethereum account address from a signed header.
-func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, error) {
-	// If the signature's already cached, return that
-	hash := header.Hash()
-	if address, known := sigcache.Get(hash); known {
-		return address.(common.Address), nil
-	}
+func RecoverSealer(header *types.Header) (common.Address, error) {
 	// Retrieve the signature from the header extra-data
 	if len(header.Extra) < extraSeal {
 		return common.Address{}, errMissingSignature
 	}
 	signature := header.Extra[len(header.Extra)-extraSeal:]
-
 	// Recover the public key and the Ethereum address
 	pubkey, err := crypto.Ecrecover(sigHash(header).Bytes(), signature)
 	if err != nil {
@@ -186,7 +180,20 @@ func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, er
 	}
 	var signer common.Address
 	copy(signer[:], crypto.Keccak256(pubkey[1:])[12:])
+	return signer, nil
+}
 
+// ecrecover extracts the Ethereum account address from a signed header.
+func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, error) {
+	// If the signature's already cached, return that
+	hash := header.Hash()
+	if address, known := sigcache.Get(hash); known {
+		return address.(common.Address), nil
+	}
+	signer, err := RecoverSealer(header)
+	if err != nil {
+		return common.Address{}, err
+	}
 	sigcache.Add(hash, signer)
 	return signer, nil
 }
@@ -341,7 +348,7 @@ func (c *Clique) verifyCascadingFields(chain consensus.ChainReader, header *type
 		parent = chain.GetHeader(header.ParentHash, number-1)
 	}
 	if parent == nil || parent.Number.Uint64() != number-1 || parent.Hash() != header.ParentHash {
-		return consensus.ErrUnknownAncestor
+		return fmt.Errorf("unknown ancestor [0x%x] for block %d", header.ParentHash, number)
 	}
 	if parent.Time.Uint64()+c.config.Period > header.Time.Uint64() {
 		return ErrInvalidTimestamp
@@ -411,14 +418,14 @@ func (c *Clique) snapshot(chain consensus.ChainReader, number uint64, hash commo
 			// If we have explicit parents, pick from there (enforced)
 			header = parents[len(parents)-1]
 			if header.Hash() != hash || header.Number.Uint64() != number {
-				return nil, consensus.ErrUnknownAncestor
+				return nil, fmt.Errorf("unknown ancestor [0x%x] for block %d", header.ParentHash, number)
 			}
 			parents = parents[:len(parents)-1]
 		} else {
 			// No explicit parents (or no more left), reach out to the database
 			header = chain.GetHeader(hash, number)
 			if header == nil {
-				return nil, consensus.ErrUnknownAncestor
+				return nil, fmt.Errorf("unknown ancestor [0x%x] for block %d", hash, number)
 			}
 		}
 		headers = append(headers, header)
@@ -481,23 +488,23 @@ func (c *Clique) verifySeal(chain consensus.ChainReader, header *types.Header, p
 		return err
 	}
 	if _, ok := snap.Signers[signer]; !ok {
-		return errUnauthorized
+		return fmt.Errorf("unauthorised[2]: signer 0x%x is not one of the signers", signer)
 	}
 	for seen, recent := range snap.Recents {
 		if recent == signer {
 			// Signer is among recents, only fail if the current block doesn't shift it out
 			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
-				return errUnauthorized
+				return fmt.Errorf("unauthorised: signer 0x%x recently signed", signer)
 			}
 		}
 	}
 	// Ensure that the difficulty corresponds to the turn-ness of the signer
 	inturn := snap.inturn(header.Number.Uint64(), signer)
 	if inturn && header.Difficulty.Cmp(diffInTurn) != 0 {
-		return errInvalidDifficulty
+		return fmt.Errorf("Invalid difficulty [block %d], was %d expected %d (in-turn)", header.Number, header.Difficulty, diffInTurn)
 	}
 	if !inturn && header.Difficulty.Cmp(diffNoTurn) != 0 {
-		return errInvalidDifficulty
+		return fmt.Errorf("Invalid difficulty [block %d], was %d expected %d (out-of-turn)", header.Number, header.Difficulty, diffNoTurn)
 	}
 	return nil
 }
@@ -558,7 +565,7 @@ func (c *Clique) Prepare(chain consensus.ChainReader, header *types.Header) erro
 	// Ensure the timestamp has the correct delay
 	parent := chain.GetHeader(header.ParentHash, number-1)
 	if parent == nil {
-		return consensus.ErrUnknownAncestor
+		return fmt.Errorf("unknown ancestor [0x%x] for block %d", header.ParentHash, number)
 	}
 	header.Time = new(big.Int).Add(parent.Time, new(big.Int).SetUint64(c.config.Period))
 	if header.Time.Int64() < time.Now().Unix() {
@@ -613,7 +620,7 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, results c
 		return err
 	}
 	if _, authorized := snap.Signers[signer]; !authorized {
-		return errUnauthorized
+		return fmt.Errorf("unauthorised[1]: signer 0x%x is not one of the signers", signer)
 	}
 	// If we're amongst the recent signers, wait for the next block
 	for seen, recent := range snap.Recents {
diff --git a/consensus/clique/snapshot_test.go b/consensus/clique/snapshot_test.go
index 17719884f0..7d14430d4d 100644
--- a/consensus/clique/snapshot_test.go
+++ b/consensus/clique/snapshot_test.go
@@ -19,6 +19,9 @@ package clique
 import (
 	"bytes"
 	"crypto/ecdsa"
+	"fmt"
+	"github.com/ethereum/go-ethereum/core/vm"
+	"io"
 	"math/big"
 	"testing"
 
@@ -42,6 +45,8 @@ type testerVote struct {
 // keys capable of signing transactions.
 type testerAccountPool struct {
 	accounts map[string]*ecdsa.PrivateKey
+	// can be used to always yield the same accounts
+	fakePRNG io.Reader
 }
 
 func newTesterAccountPool() *testerAccountPool {
@@ -49,21 +54,52 @@ func newTesterAccountPool() *testerAccountPool {
 		accounts: make(map[string]*ecdsa.PrivateKey),
 	}
 }
+func newDeterministicTesterAccountPool() *testerAccountPool {
+	return &testerAccountPool{
+		accounts: make(map[string]*ecdsa.PrivateKey),
+		fakePRNG: &fakeRand{},
+	}
+}
 
-func (ap *testerAccountPool) sign(header *types.Header, signer string) {
+type fakeRand struct {
+	counter uint64
+}
+
+func (f *fakeRand) Read(p []byte) (n int, err error) {
+	for i := 0; i < len(p); i++ {
+		p[i] = byte(f.counter)
+	}
+	f.counter++
+	return len(p), nil
+}
+
+func (ap *testerAccountPool) getSignature(header *types.Header, signer string) ([]byte, error) {
 	// Ensure we have a persistent key for the signer
 	if ap.accounts[signer] == nil {
-		ap.accounts[signer], _ = crypto.GenerateKey()
+		if ap.fakePRNG != nil {
+			ap.accounts[signer], _ = ecdsa.GenerateKey(crypto.S256(), ap.fakePRNG)
+		} else {
+			ap.accounts[signer], _ = crypto.GenerateKey()
+		}
 	}
 	// Sign the header and embed the signature in extra data
-	sig, _ := crypto.Sign(sigHash(header).Bytes(), ap.accounts[signer])
+	return crypto.Sign(sigHash(header).Bytes(), ap.accounts[signer])
+}
+
+func (ap *testerAccountPool) sign(header *types.Header, signer string) {
+	// Sign the header and embed the signature in extra data
+	sig, _ := ap.getSignature(header, signer)
 	copy(header.Extra[len(header.Extra)-65:], sig)
 }
 
 func (ap *testerAccountPool) address(account string) common.Address {
 	// Ensure we have a persistent key for the account
 	if ap.accounts[account] == nil {
-		ap.accounts[account], _ = crypto.GenerateKey()
+		if ap.fakePRNG != nil {
+			ap.accounts[account], _ = ecdsa.GenerateKey(crypto.S256(), ap.fakePRNG)
+		} else {
+			ap.accounts[account], _ = crypto.GenerateKey()
+		}
 	}
 	// Resolve and return the Ethereum address
 	return crypto.PubkeyToAddress(ap.accounts[account].PublicKey)
@@ -75,16 +111,129 @@ type testerChainReader struct {
 	db ethdb.Database
 }
 
-func (r *testerChainReader) Config() *params.ChainConfig                 { return params.AllCliqueProtocolChanges }
-func (r *testerChainReader) CurrentHeader() *types.Header                { panic("not supported") }
-func (r *testerChainReader) GetHeader(common.Hash, uint64) *types.Header { panic("not supported") }
-func (r *testerChainReader) GetBlock(common.Hash, uint64) *types.Block   { panic("not supported") }
-func (r *testerChainReader) GetHeaderByHash(common.Hash) *types.Header   { panic("not supported") }
+func (r *testerChainReader) Config() *params.ChainConfig  { return params.AllCliqueProtocolChanges }
+func (r *testerChainReader) CurrentHeader() *types.Header { panic("not supported") }
+func (r *testerChainReader) GetHeader(hash common.Hash, number uint64) *types.Header {
+	return rawdb.ReadHeader(r.db, hash, number)
+}
+func (r *testerChainReader) GetBlock(common.Hash, uint64) *types.Block { panic("not supported") }
+func (r *testerChainReader) GetHeaderByHash(common.Hash) *types.Header { panic("not supported") }
 func (r *testerChainReader) GetHeaderByNumber(number uint64) *types.Header {
 	if number == 0 {
 		return rawdb.ReadHeader(r.db, rawdb.ReadCanonicalHash(r.db, 0), 0)
 	}
-	return nil
+	return rawdb.ReadHeader(r.db, rawdb.ReadCanonicalHash(r.db, number), number)
+}
+
+func TestCliqueReorgAtEpoch(t *testing.T) {
+
+	// Create the account pool and generate the initial set of signers
+	accounts := newDeterministicTesterAccountPool()
+
+	// 7 signers
+	signerlist := []string{"A", "B", "C", "D", "E", "F", "G"}
+
+	signers := make([]common.Address, len(signerlist))
+	for j, signer := range signerlist {
+		signers[j] = accounts.address(signer)
+	}
+	// Sort the signers
+	for j := 0; j < len(signers); j++ {
+		for k := j + 1; k < len(signers); k++ {
+			if bytes.Compare(signers[j][:], signers[k][:]) > 0 {
+				signers[j], signers[k] = signers[k], signers[j]
+				signerlist[j], signerlist[k] = signerlist[k], signerlist[j]
+			}
+		}
+	}
+	// Create the genesis block with the initial set of signers
+	genesis := &core.Genesis{
+		ExtraData: make([]byte, extraVanity+common.AddressLength*len(signers)+extraSeal),
+	}
+	for j, signer := range signers {
+		copy(genesis.ExtraData[extraVanity+j*common.AddressLength:], signer[:])
+		fmt.Printf("adding signer 0x%x\n", signer)
+	}
+	sealerListBytes := genesis.ExtraData
+	// Create a pristine blockchain with the genesis injected
+	db := ethdb.NewMemDatabase()
+	genesis.Commit(db)
+
+	engine := New(&params.CliqueConfig{Epoch: 5}, db)
+
+	// Initialize a fresh chain with only a genesis block
+	blockchain, _ := core.NewBlockChain(db, nil, params.AllEthashProtocolChanges, engine, vm.Config{})
+	snap, err := engine.snapshot(&testerChainReader{db: db}, blockchain.CurrentBlock().NumberU64(), blockchain.CurrentBlock().Hash(), nil)
+	for addy, _ := range snap.Signers {
+		fmt.Printf("signer: 0x%x\n", addy)
+	}
+
+	// Generate blocks, up to the checkpoint block
+	// This segment should with the last block being of higher difficulty: in-turn
+	blocks2, _ := core.GenerateChainSeal(params.TestChainConfig, blockchain.CurrentBlock(), engine, db, 6,
+		func(i int, b *core.BlockGen) {
+			b.SetDifficulty(diffNoTurn)
+			extraData := make([]byte, extraVanity+extraSeal)
+			b.SetExtra(extraData)
+			if i+1 == 5 {
+				// Checkpoint block
+				b.SetExtra(sealerListBytes)
+				b.SetDifficulty(diffInTurn)
+			}
+		},
+		func(b *types.Block) *types.Header {
+			// Seal it
+			hdr := b.Header()
+			if hdr.Number.Uint64() >= 5 {
+				k := 5 % len(signerlist)
+				sig, _ := accounts.getSignature(hdr, signerlist[k])
+				extradata := b.Extra()
+				copy(extradata[len(extradata)-65:], sig)
+				hdr.Extra = extradata
+			} else {
+				k := int(b.NumberU64()+5) % len(signerlist)
+				sig, _ := accounts.getSignature(hdr, signerlist[k])
+				extradata := b.Extra()
+				copy(extradata[len(extradata)-65:], sig)
+				hdr.Extra = extradata
+
+			}
+			return hdr
+		})
+
+	addBlocks := func(blocks []*types.Block) {
+		fmt.Printf("Importing %d blocks\n", len(blocks))
+		for _, block := range blocks {
+			author, _ := engine.Author(block.Header())
+			fmt.Printf(" Adding block %d: 0x%x, sealer: 0x%x, len(extra): %v, difficulty %d\n", block.Number(), block.Header().Hash(), author, len(block.Extra()), block.Difficulty())
+		}
+		if _, err := blockchain.InsertChain(blocks); err != nil {
+			t.Fatalf("failed to import blocks: %v", err)
+		}
+	}
+	showRecents := func() {
+		snap, err = engine.snapshot(&testerChainReader{db: db}, blockchain.CurrentBlock().NumberU64(), blockchain.CurrentBlock().Hash(), nil)
+		if err != nil {
+			t.Fatalf("failed to generate snapshot: %v", err)
+		}
+		fmt.Printf("Recents:\n")
+		for block, addy := range snap.Recents {
+			fmt.Printf("%v 0x%x\n", block, addy)
+		}
+	}
+	addBlocks(blocks2[0:4])
+	showRecents()
+	addBlocks(blocks2[4:5])
+	showRecents()
+	addBlocks(blocks2[5:6])
+	showRecents()
+	fmt.Printf("Current head: 0x%x\n", blockchain.CurrentBlock().Header().Hash())
+	for block, addy := range snap.Recents {
+		fmt.Printf("%v 0x%x\n", block, addy)
+	}
+	if blockchain.CurrentBlock().Number().Uint64() == 6 {
+		t.Error("Block should have been rejected")
+	}
 }
 
 // Tests that voting is evaluated correctly for various simple and complex scenarios.
diff --git a/core/chain_makers.go b/core/chain_makers.go
index de0fc6be9e..0c52fd1536 100644
--- a/core/chain_makers.go
+++ b/core/chain_makers.go
@@ -128,6 +128,11 @@ func (b *BlockGen) AddUncle(h *types.Header) {
 	b.uncles = append(b.uncles, h)
 }
 
+// SetDifficulty sets difficulty of the block
+func (b *BlockGen) SetDifficulty(d *big.Int) {
+	b.header.Difficulty = d
+}
+
 // PrevBlock returns a previously generated block by number. It panics if
 // num is greater or equal to the number of the block being generated.
 // For index -1, PrevBlock returns the parent block given to GenerateChain.
@@ -165,6 +170,9 @@ func (b *BlockGen) OffsetTime(seconds int64) {
 // values. Inserting them into BlockChain requires use of FakePow or
 // a similar non-validating proof of work implementation.
 func GenerateChain(config *params.ChainConfig, parent *types.Block, engine consensus.Engine, db ethdb.Database, n int, gen func(int, *BlockGen)) ([]*types.Block, []types.Receipts) {
+	return GenerateChainSeal(config, parent, engine, db, n, gen, nil)
+}
+func GenerateChainSeal(config *params.ChainConfig, parent *types.Block, engine consensus.Engine, db ethdb.Database, n int, gen func(int, *BlockGen), sealer func(block *types.Block) *types.Header) ([]*types.Block, []types.Receipts) {
 	if config == nil {
 		config = params.TestChainConfig
 	}
@@ -205,6 +213,11 @@ func GenerateChain(config *params.ChainConfig, parent *types.Block, engine conse
 			if err := statedb.Database().TrieDB().Commit(root, false); err != nil {
 				panic(fmt.Sprintf("trie write error: %v", err))
 			}
+			if sealer != nil {
+				header := sealer(block)
+				return types.NewBlock(header, b.txs, b.uncles, b.receipts), nil
+			}
+
 			return block, b.receipts
 		}
 		return nil, nil