Revealing mistake: In the beginning scene when they jump over the drawbridge there is more than a foot of space at the bottom of the rising bridge. No car, no matter how big, could have cleared that space without flipping over. Also when they land on the other side you can see a "flap" at the bottom of the bridge which would allow the car to drive over it. (00:09:40)
Revealing mistake: After Jake topples down the stairs in the desk, the broken remnants of the desk are actually planks of balsa wood cut in the general shape of a desk. The amount of the debris changes from shot to shot. (00:14:40)
Revealing mistake: When Jake sees the light and shouts, "The band!", he flips through the church and back to Elwood. When he stops, he is almost as tall as Elwood. This is because the stunt double is not nearly as short as John Belushi. (00:22:10)
Revealing mistake: When Jake and Elwood are driving through the mall, they leave by driving through some store, if you watch as the people run to get out of the way, a black man with an apron on runs right into the camera. (00:31:40)
Revealing mistake: In the mall where the police car is spinning around on its roof (when the cop says, "They broke my watch"), you can see there's a separate circle of floor just larger than the car's roof, upon which it is rotating. (00:31:47)
Revealing mistake: When Jake, Elwood and the cops are buried in rubble from the hotel, you see one of the cops dragging himself out of the debris has two bricks sitting in the brim of his hat. That's either a very strong hat or very light bricks... (00:42:05)
Revealing mistake: During the end concert when Cab Calloway introduces Jake and Elwood, Willie the drummer isn't even hitting the drums - most noticeably the ride cymbal. (00:48:15)
Revealing mistake: In the last scene where we see multiple police cars pile up under the L-platform, at one point you can see the boards that allow some of the cars to drive on top of the lower cars. (02:08:30)
Revealing mistake: During the last chase scene, pursued by the Nazis, the Bluesmobile stops short of running off of the bridge. You can see the steel guide rails behind the front wheels which will help the car get its front wheels back on the bridge when he slams it into reverse. (02:10:45)
Revealing mistake: When Elwood drives the Blues Mobile backwards from the half-finished bridge, just as it flips you see something on the underside of the car on the far right. It's either a coiled spring or a tube with an explosive charge used to flip the car over. (02:10:50)
Revealing mistake: During the big chase scene at the end, right before the Nazis accidentally drive off the bridge, the Bluesmobile is seen doing a backflip over their car. Watch carefully as the Bluesmobile first lifts off the ground - an air-ram is visible firing under the car, which causes the car to begin the flip. (Note: This scene has been cropped in some releases so that the air-ram isn't visible, but it is still present in other releases of the film).
Revealing mistake: When the cop car hits the Winnebago to chase the Blues Brothers, we see a puff of white smoke emit from under the cop car as it rolls over that was used to help flip it. The pole can also be seen rolling around near the top of the screen in that same shot.
Revealing mistake: During the mall chase scene, a cop car flips upside down and is sliding. At one point, the wires pulling it are briefly visible.
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.
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.
Answer: It's 'Can't Turn You Loose,' often refered to as the Blues Brothers Theme.
rabid anarchist
Also done much earlier by Otis Redding.