My worst screw up: When putting in Synergy ball joints I put an upper in upside-down. Don't ask, but there was no beer involved. I thought I'd never get it out.

 

On my Chevy Blazer I had to replace a stuck headlight relay. I didn't connect the new one the right way and ended up driving with one headlight in low, the other in high beam. When activating the high beams they just switched.
I found out about my mistake when I parked it against a wall in a parking garage.
As soon as I got home (in the middle of the night) I corrected my mistake.....the screwdriver in my hand and a flashlight in my mouth.