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.
Originally, Maxwell Atoms wanted Grim to speak in a "British" accent and had Jonathan Harris in mind to voice him, when Greg Eagles auditioned and spoke in a Jamaican accent, he was impressed and had him retooled.
The show used to be called "Grim & Evil", and featured two segments of "The Grim Adventures of Billy and Mandy" and one segment of "Evil Con Carne", with some exceptions. Since then, the "Evil" segments have been removed from the show, and temporarily had his own show.
The show started as a short film made by Maxwell Atoms back in 1996. The film is about Billy wanting his inner demons out of him, and Mandy wanting to saw him in half, and Grim would emerge during the process. The film has not emerged, since it's in a film roll according to Atoms. But there is a picture of what the film looked like, Billy looks pretty much the same, but Mandy has major differences than her appearance in the show: she has pigtails, she has an evil smile, and she has a nose.