The spooky droning gibberish played in the end credits in a creepy voice is Maxwell Atoms talking backwards saying: "No, no. This is the end of the show. You're watching it backwards!"
Originally, Maxwell Atoms wanted Grim to speak in a British accent, and had Jonathan Harris in mind to voice him. When Greg Eagles auditioned in a Jamaican accent, Atoms was impressed and had Grim retooled.
The episode "Smell of Vengeance" was to have a scene where Hector Con Carne's Stinker Ray attacks the World Trade Center Towers, and the people would jump from the buildings, but the scene was hurriedly removed after the terrorist attacks of September 11, 2001.
The series was originally pitched to Nickelodeon, but was only rejected once. Atoms later pitched the show to Cartoon Network, who agreed to green light its production.
In season three, episode thirteen, "Wishbones," the truck with which the bunny tries to run down Pudin' is Optimus Prime from The Transformers (1984).