I spent a few hours one day trying to make a small adjustable DC supply regulate properly- it was built by electrical fitter mechanic apprentices in the 80s, so no diagrams, and some pretty average construction... But it had "sentimental value".
I had a rough idea of what the circuit was doing, but I couldn't for the life of me explain why the ammeter wasn't working, and the voltage regulation was playing silly buggers with me. I took careful note of a mark on the ammeter "I max" at 750mA, and was careful not to exceed that, measuring the output with my DMM.
I leant back, and asked a question to a colleague... Who walked over, grabbed the current regulation knob and cranked it out. Immediately, a large resistor caught fire, as 10A flowed from the output... The resistor had burnt a hole clean through the board, and the main transistor (an MJE3055, I think?) had gone short circuit... Needless to say, the customer was not thrilled with the report "technician error- unrepairable"...