diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index d3d45b8f09f1..4391abec7e33 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -130,7 +130,12 @@ func (dl *downloadTester) HasFastBlock(hash common.Hash, number uint64) bool {
 func (dl *downloadTester) GetHeaderByHash(hash common.Hash) *types.Header {
 	dl.lock.RLock()
 	defer dl.lock.RUnlock()
+	return dl.getHeaderByHash(hash)
+}
 
+// getHeaderByHash returns the header if found either within ancients or own blocks)
+// This method assumes that the caller holds at least the read-lock (dl.lock)
+func (dl *downloadTester) getHeaderByHash(hash common.Hash) *types.Header {
 	return dl.ownHeaders[hash]
 }
 
@@ -197,7 +202,13 @@ func (dl *downloadTester) FastSyncCommitHead(hash common.Hash) error {
 func (dl *downloadTester) GetTd(hash common.Hash, number uint64) *big.Int {
 	dl.lock.RLock()
 	defer dl.lock.RUnlock()
+	return dl.getTd(hash)
+}
 
+// getTd retrieves the block's total difficulty if found either within
+// ancients or own blocks).
+// This method assumes that the caller holds at least the read-lock (dl.lock)
+func (dl *downloadTester) getTd(hash common.Hash) *big.Int {
 	return dl.ownChainTd[hash]
 }
 
@@ -205,27 +216,34 @@ func (dl *downloadTester) GetTd(hash common.Hash, number uint64) *big.Int {
 func (dl *downloadTester) InsertHeaderChain(headers []*types.Header, checkFreq int) (i int, err error) {
 	dl.lock.Lock()
 	defer dl.lock.Unlock()
-
 	// Do a quick check, as the blockchain.InsertHeaderChain doesn't insert anything in case of errors
-	if _, ok := dl.ownHeaders[headers[0].ParentHash]; !ok {
-		return 0, errors.New("unknown parent")
+	if dl.getHeaderByHash(headers[0].ParentHash) == nil {
+		return 0, fmt.Errorf("InsertHeaderChain: unknown parent at first position, parent of number %d", headers[0].Number)
 	}
+	var hashes []common.Hash
 	for i := 1; i < len(headers); i++ {
+		hash := headers[i-1].Hash()
 		if headers[i].ParentHash != headers[i-1].Hash() {
-			return i, errors.New("unknown parent")
+			return i, fmt.Errorf("non-contiguous import at position %d", i)
 		}
+		hashes = append(hashes, hash)
 	}
+	hashes = append(hashes, headers[len(headers)-1].Hash())
 	// Do a full insert if pre-checks passed
 	for i, header := range headers {
-		if _, ok := dl.ownHeaders[header.Hash()]; ok {
+		hash := hashes[i]
+		if dl.getHeaderByHash(hash) != nil {
 			continue
 		}
-		if _, ok := dl.ownHeaders[header.ParentHash]; !ok {
-			return i, errors.New("unknown parent")
+		if dl.getHeaderByHash(header.ParentHash) == nil {
+			// This _should_ be impossible, due to precheck and induction
+			return i, fmt.Errorf("InsertHeaderChain: unknown parent at position %d", i)
 		}
-		dl.ownHashes = append(dl.ownHashes, header.Hash())
-		dl.ownHeaders[header.Hash()] = header
-		dl.ownChainTd[header.Hash()] = new(big.Int).Add(dl.ownChainTd[header.ParentHash], header.Difficulty)
+		dl.ownHashes = append(dl.ownHashes, hash)
+		dl.ownHeaders[hash] = header
+
+		td := dl.getTd(header.ParentHash)
+		dl.ownChainTd[hash] = new(big.Int).Add(td, header.Difficulty)
 	}
 	return len(headers), nil
 }
@@ -237,9 +255,9 @@ func (dl *downloadTester) InsertChain(blocks types.Blocks) (i int, err error) {
 
 	for i, block := range blocks {
 		if parent, ok := dl.ownBlocks[block.ParentHash()]; !ok {
-			return i, errors.New("unknown parent")
+			return i, fmt.Errorf("InsertChain: unknown parent at position %d / %d", i, len(blocks))
 		} else if _, err := dl.stateDb.Get(parent.Root().Bytes()); err != nil {
-			return i, fmt.Errorf("unknown parent state %x: %v", parent.Root(), err)
+			return i, fmt.Errorf("InsertChain: unknown parent state %x: %v", parent.Root(), err)
 		}
 		if _, ok := dl.ownHeaders[block.Hash()]; !ok {
 			dl.ownHashes = append(dl.ownHashes, block.Hash())
@@ -262,7 +280,7 @@ func (dl *downloadTester) InsertReceiptChain(blocks types.Blocks, receipts []typ
 			return i, errors.New("unknown owner")
 		}
 		if _, ok := dl.ownBlocks[blocks[i].ParentHash()]; !ok {
-			return i, errors.New("unknown parent")
+			return i, errors.New("InsertReceiptChain: unknown parent")
 		}
 		dl.ownBlocks[blocks[i].Hash()] = blocks[i]
 		dl.ownReceipts[blocks[i].Hash()] = receipts[i]