This isn't something that happened before a race but after the race. The dumbest thing I have done before a race was sit in the hot tub the night before state in high school. I didn't know it at the time but I ran pretty bad the next day and my coach wondered what the hell happened. I had no explaination. I somehow told him I sat in the hot tub the night before and he told me why you shouldn't do that.
But one of my teammates bought a big tin can of Nestles Quick Chocolate flavor after a race and also bought a quart of milk. Someone dared him to miss all that quick with the milk and so he did. He kept dumping Quick in the milk container and drank whatever would be ready to spill out. Anyway the stuff was so thick and goowy that it was just like soild Quick. He eat the stuff and we had it figuered out to how many calories he consumed and so forth and he was in the hurt locker for a long time. Pretty funny.