Continuity mistake: When the cop car comes out to chase the Blues Brothers from behind the sign, the car starts off directly behind the sign. When the Good Ole Boys pass in the Winnebago, we see the cop car is before the sign and already has a running start.
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.