Deliberate mistake: The shots of the prisoner skiff during the transport to the Sarlacc Pit are reversed. This can, for example, be seen as Chewie's bandolier is on the wrong side, and that Han's shirt is overlaying the wrong way.
Deliberate mistake: When they arrive in Washington D.C., they are seen in a 1976 Dodge Dart taxicab, but the movie supposedly took place in 1974. There are several scenes where many cars used in the movie are post-1974.
Deliberate mistake: The scene where Arnie takes Denis for a high-speed ride was obviously shot in a studio. Arnie throws an empty beer can out of a car doing 80+mph, so the window was rolled down - but his hair isn't ruffled at all. Alternatively, he used concrete as hairspray.