One of my early games was a Bally Cybernaut. When I bought it, I thought the head needed to be removed, so I pulled off every connector in the head before realizing the head just folded down. I moved the game home and reconnected all of the connectors and the MPU wouldn't boot. I spent probably a couple months trying to fix the MPU (it booted fine on a power supply) before realizing that there were two identical connectors that were pinned the same. I swapped them back and then realized that it burned up some header pins on some lamp boards under the playfield. Hours of troubleshooting over about 3 months before I got it fully working. Lots of lessons learned on that one.