Files transferred completely, but CRC doesn't match

[OrganicGypsy]

So I have been using BTSync for a variety of purposes and ran into my first major problem the other day. While sending a file from a computer with great internet to a computer that has very flaky Wifi Tether only internet a file arrived which was broken. Further investigation revealed that the received file has different checksum values than the original. Unfortunately, I wound up having to delete the very large file and trigger a resync by moving it into a directory and restarting BTSync on the source computer. I was under the impression that the bittorrent protocol had built in block checksums and would redownload a block if it arrived broken, which would have been much preferred to resending a 2GB file. Is this kind of error checking mechanism not present in BTSync, or does this represent something more sinister, like a statistically unlikely incident where the block checksum matched, but the content was different, or a failing hard drive on the destination computer?