Revealing mistake: During the opening musical number, right before Gary joins the large group of people for the dance, he accidentally kicks Walter out of frame. Watch closely though. His foot comes nowhere near Walter when he supposedly "kicks" him.
Continuity mistake: During the opening song, as Gary is dancing with the crowd, right before the chorus of the song kicks in, Gary spins around, with his hand to his nose like he's about to sneeze. During the first shot from the front, he puts his hand up to his nose, and begins to turn in a clockwise direction. He turns about 15° as the camera cuts to the next shot, and he has suddenly turned a complete 180° instantly between shots, and is facing the opposite direction.
Revealing mistake: After the Muppets have visited Mr. Richman they're on the street talking. The passers-by walking behind are always the same four or five people.
Continuity mistake: Jack Black gets shaving cream put on his face by the barbershop quartet during the telethon. The pattern on his face changes instantly between shots.
Revealing mistake: During the opening musical number, after the baker says "Life's a piece of pie" and Gary is seen grabbing a plate with a piece of pie on it and tossing it over his head, watch closely. It becomes obvious that this is a fake piece of pie that is attached to the plate. Similarly, right before that, he grabs a cake out of another baker's hand and gives it to Walter, and you can see he is pressing his fingers into the "frosting", yet he leaves no marks, nor has any frosting residue on his fingers, revealing that was also a prop.
Answer: He was a greedy man who wanted to drill underneath the Muppet Theater for oil. He was Ebenezer Scrooge.