Gavin receives a wedgie towards the end of the film so violent it rips what appears to be white briefs off of him completely. However, a short time later, the top of Gavin's boxers are clearly visible.
Obvious cast of the dung, since the big kid hit the dung, and it stayed the same shape. This is actually not what happened in the scene at approximately 7:10 Hobie trips and falls landing on his right shoulder just in front of the big pile of poo. He rolls forward with his face toward the ground, lifts his head a bit showing the pile of poo on the ground behind him. He has no poo in his hair or the right side of his cheek. If he had actually landed on the poo that would have left open a lot more crass jokes for the teenage boys to make. No jokes no poo on the face. (However, it still could be and probably is a fake pile of Sasquatch poo.