When Marshall McClean addresses a group of young people, including Ryan Kelley, he's first wearing an Anglican/Episcopal priest's collar and then later he's pictured wearing a Roman Catholic priest's collar.
Bobby's mother sends him a church pamphlet about AIDS (in the film's time-line, this would be 1981 or sometime in 1982). The CDC began using the acronym AIDS in September 1982, but it was some time before the term became widely used.
The seduction scene from Spartacus (1960) (with Tony Curtis and Laurence Olivier) playing on the television was not in the original release of the film in 1960. Even if it were, the version playing on television in 1978 would have a been a butchered, less-violent, re-release of the film The seduction scene was not put into the film until its restoration in 1991 by Robert A. Harris. On top of that, the audio tracks were missing or destroyed. So, Tony Curtis re-recorded his lines. Since Laurence Olivier was now deceased, Anthony Hopkins re-recorded his. Although the scene fits contextually (it's an analogy about having a "taste for snails over oysters"), it's inclusion is inaccurate.
The red convertible driven by Bobby's boyfriend as they drove up to his parents' home is a 1986 Alfa-Romeo Spider even though the story took place in the early 1980s.
Throughout the film, many modern-day (2009) vehicles can be seen, even though it takes place in 1983.