We once had an error that booked quintillions of dollars notional position with a client. It was caught the same night and the client never saw it. Here is how it happened.
An end of day aging process was supposed to look at all positions marked "old" and trade into ones marked "new". Some positions may go through several transition until all the resultant positions are "new", so the process recurses until it finds a fixed point for the day.
This day the aging process for this very bespoke trade went a bit haywire and instead of booking "-1 old position, +1 new position" it booked "+1 old position, +1 new position". Seeing that is still has +2 old position, the process booked "+2 old position, +2 new position", it then booked "+4 old position, +4 new position". This went on for 16 iterations until the auto breaks kicked in and stopped the process. A quant on watch saw the process fail and restarted it causing the positions to double 16 more times.
This was billion dollar deal that grew by 2^32 times plus now existed in old and new variants for a total notional in quintillions of dollars. It was 7pm when the quant called me over because I was the only other guy still there and by happenstance I heard of similar issue happen on my old team. I fixed the position for the night but the permanent solution was made later in normal business hours.
As an aside, when a similar error happened on my old team, the recursion continued until the process ran out of stack. That was when the 16 recursion limit was put in. (The position didn't grow huge that time because it was booking "+0 old position, +1 new position", so the growth was arithmetic and not geometric)