In the episode "The Birdbot of Ice-Catraz", where Bender causes the dark matter oil spill on Pluto, Bender reverses the Planet Express ship onto the oil tanker so he can tow it. When he does this, a reversing horn can be heard. Later on in the show, when Leela tells Fry that Bender is missing, Fry goes out in the ship to look for him. He jets of in reverse but there is no reversing horn like there was earlier. It's only a small slip-up but it's worth mentioning.