I am trying to imagine how I think I would probably behave, if I was the recipient of $120 in my account.
Me: Oh, hi! There's an extra $120k in my account, and I actually noticed. I didn't put it there. You should probably contact my co-worker, whose credit union account it probably came from. We're both shareholders, and I wouldn't want to actually inconvenience a colleague.
Them: Huh?
Me: I do want the interest on whatever number of days it was in my account. It's fair that you should just kind of eat that, even if it's at the expense of the shareholders. Be less stupid next time, and exercise reasonable oversight over what your tellers do. Or pay them better and expect them to fuck up less often.
I like to think that this is how it would go, except possibly for the "Huh?" part. But frankly, unless the oversight coincided with an automatic monthly statement, I probably wouldn't notice the extra money in the first place. Massive deposits don't trigger an E-mail notification, so excess money could be in there for weeks without me noticing. I suppose that if there were an overdraft, they'd tell me. But an extra deposit? Probably not.
Do each of you, personally, have a way to notice if six figures of extra greenbacks are dumped into your account?
I go now to check my account balance. You should, too. I'm now tempted to go try to prove a negative.