I once fell (well, more kind of slid) down a set of stairs in a club & ended up with a sliver of broken plastic pint glass embedded in the back of my head that I totally failed to notice. The bouncer had to point it out to me.

Hurt like crap when the nurse pulled it out tho'

i knew a guy years ago who was jumped by a group of lads, but managed to fight them off - it was only when he got home that his mum noticed the chisel buried in his ribs, just under the scapula.
luckily, it missed any vital organs, but it was still a bit of a shock.
if it wasnt for the fact that he was such a big lad (16 yr old, 6'2, built like the proverbial brick out-house because of all the rugby he played) he would have been seriously injured, if not killed.