Spent a couple hours last night installing new brake levers, cables, bar tape and organizing my Di2 wire routing. Got everything perfectly dialed in only to realize this morning that something was amiss. I flip-flopped the brake levers!!! Now my right lever controls front brake. Talk about not paying attention.
Since this is a recipe for disaster because I'm not used to that arrangement (English style?), it's back to the basement I go.
Beyond that, I have two more good examples. Installed new chain but failed to thread it through the front derailleur cage. Whoops.
And then the expensive one. Before I was old enough to have a clue, I ordered one of those pricey ITM 'THE' Magnesium stems and installed without using a torque wrench. Funny, those are really important when dealing with expensive, lightweight parts. Immediately snapped it. When I pulled the screw out, all of the material came with it. $250 down the drain.