Other mistake: When Ned and Homer's raft is going fast, Ned decides to go on the left side of the gorge, so he paddles on the left side, which causes a raft to turn to the right, which 1, means the raft moved to the left, which is impossible, and 2, he wanted to go to the left, but paddled like he wanted the raft to turn right.
Continuity mistake: When Lisa is watching tapes of Francine's bullying, there is a scene where they are at the pool. Francine is wearing nose clips in this scene. But when Lisa freeze-frames and leans in to closer inspect the shot, Francine's face is shown on the screen without nose clips. They appear again when Lisa exclaims "Of course. The nose clips."