"MtGox remembers a hash of that transaction (unique fingerprint of its contents)" Technically, there might be collisions with hashes, they're in no way unique.

"MtGox remembers a hash of that transaction (unique fingerprint of its contents)" Technically, there might be collisions with hashes, they're in no way unique... Though the probability is rather low. Anyways, this is a nice explanation of the real issue.