I'll a throw a few at random:

1. Using a wrench to engage a crank extactor. (or a pedal in a crank, or any tapered part for that matter)
2. Washing bike with a pressure hose.
3. Lube with WD-40, or motor oil.
4. Mount a tire from the valve and moving away towards the opposite side of the rim.
5. Using levers to mount tires. (maybe with the exception of mtb's)
6. Fixing a flat without searching for the cause of the flat.
7. Mounting bar tape from the top and moving towards the bar ends.
8. Let sand and dirt build up on brake pads.
9. Using inappropriate tools, especially for very specific jobs (go to ParkTool, self-explanatory).
10. Squeezing an aluminium frame in a repair stand clamp. (this hurts just to type it )
11. Mounting Shimano pedals on a full Campy bike (just kidding ).

ok, who's turn?