Revealing mistake: As trooper Car 10 drives along in the mall, they hit an obviously poorly disguised ramp that disappears after use. Later, it's sent into a spin by unit 70, and you see it on an obvious turntable.
Continuity mistake: When the Blues Brothers first go into the tax building in Chicago, the building has gold numbers above the door that they are blocking from the inside that read 118. When the rest of the cops and a firetruck pass by, we now see the numbers are black and not even the same shape as the gold ones.
Revealing mistake: When the Chicago police cars have the first major pileup underneath the elevated roadway, a number of the cop cars are seen flipping over by using two rail-like beams along the road. When the rest of the cop cars come to a stop in the next shot, the beams are gone.
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: 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.