God there's so many pinball related things I've done wrong while shopping a machine its hard to pick. I almost always cause some sort of short in a GI string. Shorted lamps have burned up transistors on circuit boards. Dropped solder on a socket and shorted the string. Ugh. Backwards kicker switch wiring, little bits of hardware in empty lamp sockets burning when the machine is on, putting target decals on my machine, putting playfield glass on my tile kitchen floor to clean it and it exploding in my hands when it got dinged, paying 2100 for a dingy t2 when I was new to the hobby, and countless pointless drains while playing that were me just not paying attention for a second. Honestly don't regret any of these either. I almost always have to learn the hard way!