One time I was moving a Sys7 pin from one building to another. Easy enough job, pull the head, move the pieces, put the head back on, reconnect it, turn it on. Except when I turned it back on I let the magic smoke out of everything on a game that had been just fine an hour ago.
Turns out on Sys3-7 games the high voltage playfield connector and the low voltage playfield connector are the same size and shape. They are supposed to be color coded but frequently they messed that up at the factory or just made them all the same color. I didn't match up the wire color. Instead I fed the solenoid current into the logic boards and blew the top of half the chips on my driver board and some on the CPU board.