A sad circumstance of endurance events, the fudge slide happens when a competitor has to crap while on course. Instead of stopping they just let it slide down their leg, thus the name. This normally occurs with the leaders, since they want to win. Those further back will keep their dignity and go in a porto-john.
Did you see that guy that was leading in the marathon last weekend? He rode that fudge slide all the way across the line!