Probably the worst was not by me but a gentleman I sold a Krell Ksa-300 to. During our conversation before he bought it I asked him what type of connections for the signal he used and he said he has always just used single ended and I suggested to try balanced. Well after he received the amp he was experimenting between the two and when he went to switch cables he forgot to turn the amp off which resulted in blowing something in the output. Mind you this amp weighed 180# so shipping it to Krell for repairs was no easy feat. I really felt bad for the guy but it shows how a very simple mistake can end up with very bad results.