Dave's transistor outline story brought back a memory. I hadn't been working as a repair tech for long when I had the dreaded amplifier on the bench. I was doing some checks, when it began to make a horrible noise. The boss yelled at me to cut the power, but I got fumble fingered and couldn't do it in time - there were flames.
I cleaned the (not badly damaged) board, checked all the transistors and replaced the bad ones. I checked everything I could, but the thing wouldn't work. It was a direct coupled amp, and I had an offset voltage on the speaker outputs. Finally, the boss had to look at it, and I had put a transistor in according to the silk screen, and not as it should have been. I learned that lesson the hard way. That's all it was, I had indeed found all the bad parts - it was my poor re-installation.