It depends on how much came out. To clarify: Is it an amount that the IRS might be able to track if I don't go through the proper channels to keep it under wraps? If so, I have to weigh the net gain over the hassle and risk of keeping it.
Basically if the money isn't coming from a person's or business' account, and is merely hacked, and there is no surveillance on me, then why not? Especially if it's a major bank. Say, for instance, I go to withdraw $60 from an ATM at Bank of America, and it spits out $600 instead. Nobody knows I have the money, the loss isn't exactly going to sink B Of A, and if I know for sure I'm not robbing a third party, then keeping the money might start to look more appealing and not weigh on one's conscience so much. See, (in the U.S. at least) that money is FDIC insured. Never mind that that particular bank has been has been a total bastard towards its customers, they won't lose shit anyway. The money is guaranteed against loses up to $250,000 per deposit by the federal government. You've already been paying taxes to compensate banks for the very loss that you just inflicted on them.
It's hard to put this outside the hypothetical and am not even sure exactly what I'd do, but I think many wouldn't find themselves so morally righteous if they were poor and would see this as a windfall. Or, blackjack and hookers, ho!