During the opening credits, Amy, Hermes, Zoidberg, Kif, Zapp, and URL, among others, can be seen riding in the tubes.
Leela's biological father is a mutant; one of his traits is that when he speaks his mouth appears to run vertically and open horizontally, whereas it is the opposite for a "normal" person. The problem with this is that he goes back to having a normal, human-looking mouth when not speaking.
During the opening, old 1930's cartoons are played on a giant monitor and are different each time. In the episode "Roswell That Ends Well", the cartoon featured in the opening was played in the episode on a small TV. Also when the title appears, a new message appears every time.
Originally, one episode had a scene where Fry pulls down his pants and moons the business magnate Mom (of Mom's Friendly Robots). Her original dialogue was, "You call that an anus?", but it only appeared in closed captioning and was redubbed with "You call that a pressed ham?".