OK hands up. I once reassembled the crancases on a pre unit Bonneville but forgot to put the crankshaft back in.
Car wise I recently undid the top nut on front suspension strut on my lads car to replace the coil spring. Wondered why the nut seemed a bit tight until it came of the thread sending the top spring retaining plate at some velocity straight into my face. Got to admit that my blood ran a bit on the chilly side as this has been known to kill before now !. Waited for the blood to come pouring out of my nose and eyes but it didn't happen. Quick change of underwear and mental note not to forget the spring clamps next time !!!
Tin hat on
[Edited on 4-3-2006 by Paul James]