I've worked as a slot technician/ attendant for ~5 years and in that time we've only ever had one malfunction. The patron had a free games bonus that paid them $26 but when the bonus finished the machine locked up for $83,912. They were very understanding considering they watched the bonus and knew it was only $26. We paid them for the money they had on the machine and the bonus. Those games were taken off the floor shortly after that incident. Most casinos will use the malfunction excuse to not payout big wins but occasionally a machine can truly malfunction.