When Ellen discovers the microphone taped to Jim's chest, she rips it off leaving a slight bald spot on Jim's very hairy chest. However, as the scene progresses, the bald spot disappears, the hanging microphone changes position and absolutely no hair appears on the, perfectly "X" shaped, tape that was once stuck to all that hair.
In one of the first scenes, it is stated that the year is 1977, yet Ellen is shown to be driving an Oldsmobile Cutlass Cruiser that can only be as old as 1989. This is a massive oversight as the Oldsmobile Cutlass Cruiser didn't exist at all until 1982, nor did any GM vehicle of that body style as the A-Body did not exist until 1982 either, let alone the updated version from 1989.
When Jim and John are driving to the deal, we see a red Pontiac Fiero in a body style not available until January, 1987 and a white Ford F-150 utility truck is of a body style not offered until 1992. The events took place in the fall of 1982.
When you're doing a period movie, you need to ensure the cars being filmed actually existed. At the 1:25 point of the movie as Hoffman and Delorean drive down the highway with the FBI following, a black Buick Grand National appears in frame behind Hoffman's GTO. The Grand National was made from 1984-1987 and wouldn't have existed in 1981 when this movie is taking place.