Revealing mistake: In the big impressive car crash, it is quite clear that most of the cars are empty.
Audio problem: When the Blues Brothers are beginning their song Turn on Your Love Light for the battle of the bands, a horn is heard as an indicator for each singer to turn around, but none of the 3 horn players are actually playing their instrument.
Continuity mistake: When Elwood, Mac, and Buster arrive at the Monster Truck show to sing. We see Mac get out of the car. If you look carefully in the background you can see Elwood already going up to the stage. Then the camera switched to show Elwood getting out of the car.
Factual error: At the Russian funeral, the priest mentions the deceased's name as Nicholas Tsusefskaya, and describes him as a young man. The surname, however, is female. (The correct male equivalent would be Tsusefsky.).
Continuity mistake: Near the end, as Paul Shaffer judges the Battle of the Bands, the red stage curtain is closed and the stage is empty. We know that the Blues Brothers were the last performers, and they carried their microphone stands with them as they retreated behind the curtain; so, all sound gear and both bands are behind the curtain, and the stage is empty in front of the curtain except for Paul Shaffer. As Shaffer announces the winner, he ducks to his right and goes center-stage, where the curtain is instantly wide open again with band members and sound gear filling the stage all the way to the edge, which would be physically impossible to accomplish in the split-second camera cut.