[JRogers]

and i never ever mount a new rear tire on in the proper direction no matter how hard i analyze it...

I think I've done that one about 50 times. I look, double check, put the tire on and it's wrong! WTF?!

Other things I've done:
snapped a bolt while installing a new bolt-on arch on a Z1
forgot my right and left and turned the bleed screw the wrong way on a Hayes caliper and f'ed everything up inside
almost cut my finger off in a spinning disc rotor
forgetting which side of a pedal or bb is reverse threaded and trying to "loosen" it the wrong way
put a fork back together only to notice random leftover parts
repacked and adjusted a cup/cone bearing and noticed balls still on the counter after I finished

I'm sure there's more....

 

[el_gordo]

Ditto on the bottom bracket. Spent a week trying to get the reverse side out when I first started doing my own bike work. Used 4 foot pipe breaker bar on a wrench, so, of course it turned, but just a little. Thank heavens it was an old Specialized Hardrock. I finally asked at the shop, and of course their first question is "which way are you turning it???"

Last week I used the wrong crank puller -- the Park one in the toolbox has a smaller "floating" head than the one needed on a Isis type BB. Of course, the end came off and got stuck _inside_ the bottom bracket. Banged it out from the far side with only a little tweaking of the inside threads.

I've also come to the conclusion that when they say "remove the wheel and pads" when you are bleeding disc brakes, it really is for my benefit. Now, if I could just keep the brake fluid from splashing in my eye...
Gordo

 

[Knobularlife]

:eviltongu

I caught a newbie mech putting triflow on some brake pads because he could get them to stop squeeking!

:eviltongu IT was you

 

[Knobularlife]

I once dropped my samich in the parts washer

 

[JRogers]

Now, if I could just keep the brake fluid from splashing in my eye...
Gordo

Oh, I forgot about that one! I was bleeding some Hayes and, while messing with the caliper, the fluid filled tube on the caliper bleed valve came off and splashed DOT fluid directly into my eye. Spent quite some time under the sink after that.

 

[jimmydean]

Thank goodness for sealed bearings. I can't count the number of times I put headset bearings in wrong.

When I was a kid, I installed my new bottom bracket with "household oil" because I didn't have any bearing grease. It's didn't work so well.