about 3 times, once the output transisters failed in my elderly Trace Elliot, twice when signal to the sound engineer mysterious disappeared, no fault found with the amp later but I always have a spare available, but as I said I'm using elderly Trace amps.
Another amp failure story, our guitarist orange amp wouldn't switch on, he finished up using my spare Trace Elliot (sounded ok according to the crowd but he was not happy). When he took it in for repair it was just the warning light bulb that had blown, the amp was working ok all the time