This was dropped in a376b6607f, as improvement in dust checking.
Now that dust-checking is done, we still need to check if the sender has the minimum value, as decrease balance just clips to 0.
See be86f966f8 for older dust-creation problem work around, which was dropped in the above.
The bug enabled you to transfer your full balance to someone else, and pay the same amount in fee, possibly to a puppet proposer to collect back funds.
Effectively enabling printing of money. Silly bug, good to fix and introduce tests for.
d587c4fe61