Not sure i should own up to this one but this is why i'm not allowed to work on customers bikes!!
Rebuilding my race bike for the first race of the season I had the cases apart putting in the new crank after having it all balanced. Steve had shimmed the crank, gearbox etc and left the parts led out in the engine room, for me to safetyclean, airline, dry etc (a dull and time consuming job but one i'm qualified for!).
All parts spotless and led on the bench with shims in order, so i decided to put the bottom end together, which i did perfectly! sealed the cases and went home for the night.
Next morning Steve pointed out, between laffs, that he would split the cases for me so i could clean off the 3bond, then he could put the crank back in - THE RIGHT WAY ROUND!
I'll never live it down.