Failure has been my main teacher. Some highlights include:
Tinkering inside a turned on machine with no shoes on, using metal tools, and the coindoor is closed.
Incorrectly removing fuses causing them to shatter everywhere.
Plugging in a connector just because "it seems like it should go there..." ZAP!
Trying to remove a plastic without following the order of operations to get it off smoothly. Sorry plastic
Using krazy-glue for various fixes.
Lifting a playfield without removing balls. For some reason this lesson still hasn't completely sunk in.
Putting gold (brass) balls in a machine. They look cool for a few plays and then rapidly tarnish and turn everything else black.
The best of the best though happened just the other day. Decided to move my ToM about 75ft out the house and into the garage. Still don't have proper tools for moving heavy things around and did it by myself with improper tools. About 10 feet from the destination my sketchy wheels began to tip and..... TIMBER!!! The whole pin fell on its side and everything in the coinbox exploded all over the guts of the machine. To my amazement the only damage is some minor wood damage where it landed, hardly notice it and could easily be touched up.
Summary of the lessons I've learned so far:
Always wear shoes.
Turn your machine off when working on it.
Take time to do the right steps. Shortcuts always become longcuts.
As an amateur, always assume your hunches are wrong or will lead to you breaking more things.
Ask more experienced people for their opinion before you break things even more.
Use proper tools, no seriously, use them.
Never try moving a machine without help and proper pinball moving equipment.
Remove the damn balls before lifting the playfield.
No gold balls.