Worst one I had was a Gottlieb Jet Spin where things were just working strange. Narrowed it down to that somebody had 'fixed' something by moving and resoldering a bunch of wires onto the wrong lugs on a relay. This was before the days I could go on the internet and get schematics plus I was a real greenie back then, it was all by eyeballing so now I dunno how I ever figured that damn puzzle out and got it running correctly!