Corrected entry: When the truck is straddling the gap of the halves of the drawbridge in the beginning, the rear axle falls off and rolls down the slope. While funny, this is mechanically impossible.
Corrected entry: When Connie says "I think that truck..." there is snow on the ground and on the car, but in the scene of the road (after the scene when Deke is blowing bubbles), the snow is gone. This movie takes place in June, so there should have not been snow in the first place.
Correction: It is dirt on the ground not snow.
Correction: This is an absolutely possible mechanical failure. When the momentum of the rotating mass of the wheels is suddenly stopped by coming into contact with the bridge it sheers the bolts off the springs and the continued rotating energy backed by the momentum of the weight of the axle could easily break the universal joint in turn separating the axle from the truck body. Failures like this can also be seen in drag racing where sudden changes in directional motion are extremely prevalent.
I stand corrected. I should have remembered from High School Auto Mech class. Thank you.
Movie Nut