The cigarette that Shad hands Freddy in the nightclub is almost fully burnt, yet when Freddy takes it is full.
When they go to their first gig, they have all their instruments on the roof of their car, but when they leave, there's nothing on the roof.
All the musicians shown playing in the film are very obviously miming.
When Joe's new girlfriend gets out of the bathtub filled with red paint, she is very obviously wearing a wetsuit (look at her neck in closeup shot)