Here's my two dumbest things...other than these my mistakes haven't been memorable.

1 - put a new chain on and when I ran it through the rear derailler wasn't careful...ended running it OVER that little tab next to the jockey wheel instead of UNDER it. And then proceeded to ride 9 miles with it like that. Surprisingly it worked ok - slightly noisy and shifted a little poorly but not nearly as bad as I would have expected when I saw what I did wrong.

2 - tried to adjust the front derailler on my wife's bike two days before a triathlon. Just could not get it tuned right. She was not pleased. Ended up taking it down to the LBS where they showed me the cable was coming over the anchor bolt from the wrong side.