Just saw this one from Brian Fanner
Made me think - what's your worst DIY boo-boo?
I've two - bought someone else's headache and spent forever trying to fix their elementary error of getting the neck depth wrong.
The other was not remembering that the active bass comes in two flavours, one where the pickups are passive + preamp (common) and the other where the pickups are directly powered off the 9v battery (uncommon). Guess who got confused and pulled wires willy nilly. Someone else had to rescue me with parts. Facepalm, I've still not lived that one down.