I also do most of my learning from my mistakes.
I've done most of what has been mentioned in the thread so far.
I've worked with screwdrivers everyday for a lot of my life, I have the habit of when I pick them up, I grab right below the handle picking them up by the metal shaft. Well? I seem to keep doing that with the soldering iron too! You would think I would learn after a few times? Nope!
Working on the older displays plugging them in while the game is on? Yep!! I've been shocked a few times since I like to put my thumb on the back of the board while pushing the connector on. Haven't learned there yet either!
Not paying attention? Plug a Rom in backwards...I look up in the backbox trying to find the problem..."Hmmmm what's that little light on that Rom? I've never seen tha.......OH SH%% That's not a light! It's backwards!" Yep! The little window will glow if they are plugged in backwards! 
Learning resistors, I replaced all of them in the mpu almost done and realize? I'm figuring the multiplier wrong! I take them out start all over......To only get part way through? And realize? I had it wrong again!!!!! Take them all back out! I have decoding resistors down now from that lesson!!
Replaced connector pins, the type that mount to the boards. Plug my connectors on! Everything should be good!!!!! Stuff still isn't working right! I checked wires etc.....To realize an hour later....The pins I put on the board were too long, when I was pushing the other end of the connector on, it was pushing all my little wires out of their slots on the connectors I was pushing on!!!!! I didn't notice at first because of the little slide covers on the top connectors. Yep!! Different length pins make a difference!
Checking continuity on traces? I couldn't figure out what was going on? Most of the traces were connected, pick the board up, look it over, lay it down, check again, Now different traces are connected! WTH!!!! Yep! I was laying the board on a spiral notebook while checking traces!! It finally hit hit me, when I sat back in the chair to take a breath. Looked down and thought..."you freaking idiot!!!! How can you be that stupid" I was ticked when I realized I was that dumb, when I noticed the spiral notebook that I was sitting the board on.
I can go on and on.....I'm like the fumbling mad scientist of pinball, screwups!!!
