Large Dragon at Large - S2-E10
Continuity mistake: To catch Ripley, Fred uses the remote to bring the draw bridge up. But Ripley manages to scale the bridge before it gets all the way up and falls into the moat below. As he surfaces, the draw bridge is instantly and suddenly down all the way again as the gang rush out onto it. (00:18:25)
Answer: Because of the goo that was on it.