Once upon a time in the dark ages of IT a Dutch bank made a mistake in their interest calculations. One alert customer noticed the error and notified the bank but that was not before 100's of thousands of people received papers with the wrong amounts on them (in favor of the bank). That took a while to correct.

I worked at one FI in Australia that shipped a bug that impacted interest payments, except in this case they used the wrong 'from' date: the account opening date instead of the last interest date.

Some long-standing accounts received very large cheques (it was the 90s, so yes, cheques).

The FI gave the customers the choice to either return the cheques or have future interest payments withheld. Surprisingly the vast majority simply returned the cheques.

We had to write additional code to clawback the interest on the other accounts. I doubt if the FI would have had any recourse if people had just closed their accounts and kept the cheque but to the best of my knowledge nobody did.

At some point they stop being entertaining and you just want to forget :)