diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index b6f5760d9c..4db689f73b 100644
--- a/eth/downloader/downloader.go
+++ b/eth/downloader/downloader.go
@@ -1491,6 +1491,10 @@ func (d *Downloader) importBlockResults(results []*fetchResult) error {
 		if index < len(results) {
 			log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
 		} else {
+			// The InsertChain method in blockchain.go will sometimes return an out-of-bounds index,
+			// when it needs to preprocess blocks to import a sidechain.
+			// The importer will put together a new list of blocks to import, which is a superset
+			// of the blocks delivered from the downloader, and the indexing will be off.
 			log.Debug("Downloaded item processing failed on sidechain import", "index", index, "err", err)
 		}
 		return errInvalidChain