Just got an urgent call to the sales floor - a laptop had started making an incredibly loud whining sound and was a) scaring people who thought it would explode  and b) hurting their ears. I told them to turn it off and Id come up. Got up there, turned it on nothing until windows opened then it made the loudest most horrible sound ever!! It was clearly feedback so I just muted the mic and it stopped but god did we take the piss out of the sales guy!!