If there was a bug the exchange should absorb that out of their funds rather than charge customers
For the most part Vern did eat the lost coin. For all small few it was more than a couple satoshis. Same thing would happen at your bank if you got lucky and they forgot to charge you eventually they WILL charge you.
In this situation it was more like an ATM where you just keep taking out money and thinking oh I am good this is legit I musta had triple the balance I thought to begin with.