Continuity mistake: After the cop car hits the Winnebago, we see the car's back window is completely gone in two separate shots. However, when the camera angle does a close up on the car, we see a cop pushing out the rear windshield which was simply not there the shot before.
Continuity mistake: At Chez Paul, when Jake and Elwood first meet up with Mr. Fabulous, who is talking on the phone, there is a shot of them walking down the hallway towards him and you can see that Mr. Fabulous is looking right at the two brothers. In the next shot, his head is down and he then looks up at Jake and Elwood.
Answer: It was a double, and the "making of" documentary on the DVD shows this.
David Mercier
To be fair, John did do a lot of flips and dancing as Jake; just not all of them in the movie.