Continuity mistake: When talking about El Chupakneebra with the Sewer People, the characters are standing in front of a building with a blue sign reading "Cleaners". But in the shot where Leela explains how Nibbler loves fresh crocodile, the writing on the sign changes to a completely different, much narrower, style.
Continuity mistake: When the mutant with the big head first mentions El Chupakneebra, in the background you can see the blue sign reading "Cleaners". It is then attached to the building next to the one the characters are standing by. However, in the next shot, when Leela asks what El Chupakneebra is, you can see the blue sign on the building they are in front of.
Plot hole: In the sewer, the guy with the big forehead starts singing with his guitar and a crowd gathers around. But you can see Leela's parents standing in the crowd and having no reaction to their daughter whatsoever, despite them knowing what she looks like. Considering later episodes, this is a bit confusing.