Happened to one of the three 777heaven they had in our arcade, you'd hear a little 'sizzling' sound bit like a short circuit and walk over and see the machine in credit.
Used to play them and quickly collect any winnings and move away. Needless to say it didn't last very long before they fixed it, 2-3 days tops.
Management were probably feeling quit smug and then the 10p trick came along lol
Edit: Forgive me but basing this on 25+ year old memories, wasn't there a little 'trick' on Nudge XS where if you got a win other than the JP with the bonus overlay symbols (say 3 bars with 'bonus' on them) you could hold a combination of buttons such as cancel and a hold button and it would spin the top reel to the highest possible (£2.40 on the one I remember) so you'd get £1.40 for the bars plus £2.40 for them being bonus bars???
Let's see how good my memory really is, were the bars double width and white/blue in colour?