This is only marginally related, but MtGox should add a few digits to each account number representing a checksum... for example the last 4 decimal digits of the SHA256 hash of their fully-decorated MtGox account number (as it would be required in the notes of a wire transfer). And then always validate it with incoming transfers, if nothing else, to prod them to ask questions before any chance of posting it to the wrong account.
This way, the odds of submitting the account number "2" and having it be a valid account number by accident would be greatly reduced.
They already do that, and we mostly implemented it, but then all account numbers people think they know would fail.