The combination of my bike lock which I let my friend know so he could borrow my bike to cycle home for a shit at lunchtimes. Such trips were vital to him as he was desparately paranoid about catching aids or gay from the school facilities.
One day, however, I changed my combination and neglected to tell him. The first period after lunch, he stormed up to my desk with his face wet with tears. Slamming his fists down, he screeched "Thanks a fucking lot, Green, I had to shit myself today".
My astonished response was never heard, as it was drowned out by the laughter of some 20 other pupils. A cautionary tale for anyone willing to take responsibility for the toilet habits of others.