Just noticed this on episode 412: 4th Grade:
After Timmy's wheelchair goes bonkers, the two guys that biult it are on TV, and the blond haired one makes a comment about there being 72 original episodes of Star Trek, and the fatter brown haired guys says "No, there were 73." Then later when the biys go to see them to build another time machine, the blond guy says there are 73, and that the brown haired guy (can't remember what he calls him) says its 72. They messed up...
Anyway, Pete