The best one is getting a dead body round the prop, as happened to a fellow boater in Farmer's Bridge Locks 2 years ago. We were stuck overnight in the lock pound between locks 2 & 3 because the police reckoned someone had partaken of one too many pints of cider & slipped into the cavernous depths underneath the PO Tower. Guess what we have renamed Farmer's Bridge Locks now?

Close second is someone's cratch cover round the prop. You can't cut that off so easily!