Other mistake: As Fozzie corrects the car to keep from crashing, watch the two of them in the long shot by the church. They are bouncing very rapidly as they come to a stop, suggesting the film was sped up. Also, Fozzie looks to the left of the steering wheel to shift gears, rather than the right, where the shifter would be, on the transmission hump.
Continuity mistake: When Kermit and Fozzie meet Big Bird, their car stops about fifteen feet behind him, at the end of some dark tire tracks. But when the camera looks at the pair in the car, they're looking directly at their left, and Big Bird converses with them looking to his left in quarter profile to them. When they leave, their position is about ten feet back from the end of the marks, a good twenty feet from where they originally stopped.
Continuity mistake: After Kermit is tossed over the crowd in the El Sleazo, he catches on a ceiling fan, and it swings a bit from the impact. After a shot of the bartender speeding up the fan, it is no longer swinging. This is impossible with any weight on one blade. When Kermit is slung off the fan, you can see the fan is not on at all, Kermit is at the piano, and just about everybody in the place is still looking at the fan.
Continuity mistake: The crack in Fozzie's Studebaker windshield comes and goes throughout the film.