When the two are driving to the concert at the end of the film, one of Michelle's earrings is missing. When they arrive at the venue it has reappeared.
In the first scene of the girls on Hollywood Boulevard, dressed as hookers, walk by a place with "DVDs" on its sign. The scene is set in 1990, while the first DVD players did not go on sale until November 1996.
Ashley Schwartz talks about the hot new star of 21 Jump Street (1987), Richard Grieco. The movie is set in 1990, and in the previous year he'd left the show to star in his own spin-off Booker (1989) (which was canceled after one season).
Romy asks Michelle which star of Beverly Hills, 90210 (1990) she prefers - the show didn't start until autumn of that year (Pretty Woman (1990) was in cinemas earlier in 1990).
The movie takes place in 1990, yet when Romy and Michele see the ocean for the first time there is a shot of the Santa Monica pier complete with all the rides, none of which were added to the pier until 1996.