When Mrs. Murphy sings "Respect" at the Mercedes dealership, there are lots of people on the bridge outside. After a close-up, a second later in a shot from the same angle, no people at all on the bridge.
When Elwood Blues, Mighty Mack McTeer and Buster Blues sneaks into the Monster Truck Stadium to perform, the police stop their car to look inside. When it shows the police point of view, Buster is in the back of the car and Mighty Mack McTeer is in the driver seat. In the next take, Buster is suddenly in the passenger seat.
During the chase scene after the Russians torch the club when the tacks are used and the Russians car flips over, it flips over before it reaches the left side of the club and ends up in front of the club. In the previous shot the actual chase started in front of the club and they were driving fast and would have been well away from the club when the car flipped over.
When the Blues Brother's band begins to play for the actual "battle of the bands" at Queen Mousette's, you can hear a Baritone Sax playing, but Blue Lou Marini is holding a Tenor Sax, and not actually playing it at that moment.
During the scene where Elwood spins his car into the parking spot in front of Willie's, the space between the cars to the front and rear are smaller after Elwood leaves the car. Yet during the scene where the car is spinning into place, the gaps between the cars is much greater.
Just after Elwood starts the Bluesmobile by radio control, he uses the controller to move the car, and is seen turning the steering wheel to the left. However, the car is shown making a *right* turn as it leaves its parking spot.
While sifting through the rubble of the burned down night club, the police find a pile of shell casings from an assault rifle, but the Russians were firing sub-machine guns (Uzis and Mac-10s), which use pistol ammunition.
After the Russians torch Willie's club and chase the Bluesmobile, Mack throws the nails out of the window. At the point where the Russian car's tyres blow out, the ramp used to flip the car over is clearly visible.
The Indiana State Police cars that first appear outside of Bobs Country Junction while the band is inside eating have a front plate, Indiana automobiles only have one plate; on the rear. Furthermore the uniforms worn by the Indiana State Police officers in the movie are incorrect. They have never worn green and never worn a jacket. Further along in the movie after the brothers are spotted at the restaurant and during the road block on the bridge the correct blue Indiana State Police uniform with upside down triangle patch (probably worn by actual officers who were called in to be extras) can be seen in the back (this is just after one of the troopers wearing the green uniform finishes talking to the person playing the FBI agent.)
When Elwood meets Cab for the first time and tells him about Curtis, Cab says that his mother lives in Phoenix, Arizona and then calls her. He is in Chicago and only dials 7 numbers when he needed an area code.
When the sniper is lining up his shot at the truck rally show, the front cap on his scope appears closed. However, this is a night vision scope that has a protective cover that allows only the right amount of light in as to not ruin the light sensor inside the scope.
During part of the "Ghost Riders" set, there is a cut to the sniper's infrared sight, which shows Elwood on stage using the remote control for the car, and he has his mouth shut at the same time that we hear singing. However, the singing we hear comes from Mack, who is off-screen. Elwood and Mack sing alternate lines during this section of the song.
When the band leaves for their play date in Louisiana, they have only two cars. The passenger compartments of both are filled with the band members, and the trunk of the Blues Mobile contains the young member Buster. This leaves only the trunk of the other car to carry the band's equipment. It would be impossible to fit all their guitars, amps, horns, drums, and other gear into this one small trunk.
When the flammable liquid used by the Russians to burn Willie's ignites along the vertical surfaces of the bar, it is obvious they are using pyrotechnics gel as the flames are seen clinging to the vertical surfaces. Liquid fuels do not burn like that since they will merely run down the surface.
As the Bluesmobile is driving up out of the river (with the fanatics' boat on top), one can just barely make out an optical effect just under and ahead of the car, roughly aligned with the water pouring off. This was probably the result of a camera trick used to hide a tow cable.