To check fuses with a multimeter when stuff isnt working. There was a fuse that looked whole and i spent weeks working on my indiana jones taking the mini playfield apart over and over (not an easy task), buying a new motor, spending money on a board. A $2 fix became weeks and $200. My brother came over and fixed it in 2 seconds.

It was early on before I really knew what i was doing but ive had several lessons that cost me money because i didnt understand how to remove something or check something properly.

Amen to changing bulbs with power off. Ive actually damaged my mpu by accidental shorts when doing this aside from popping fuses. You always think... Oh, ill be careful, until you drop something or reach for something or hand slip.