Sister Assumpta's Irish accent disappears.
When Tim and Francis are walking with the dog, a truck can be seen behind them in one shot but not in the next. It subsequently passes right by them.
In the scene where Donnie is fighting Francis, it is clearly visible that it is in fact Francis. But when it zooms in on Donnie punching Francis, it is someone else. This boy has blonde hair instead of black hair. Than as the boy rolls over, you can clearly tell that it is Francis and his hair is back to it's normal color.
The poem quoted in the funeral is found in William Blake's "Songs of Innocence" and "Songs of Experience", not in "Marriage of Heaven and Hell". (possible character error?)
(at around 22 mins) Tim says that cougars are an endangered species. This is incorrect. Cougars are abundant across North America and is listed as "least concern" on the threatened species Red List.
A G.I. Joe comic book is seen in Sister Assumpta's drawer of confiscated items, but G.I. Joe comic books did not appear until 1983 (after the film was set).
Movie takes place in the 1970s but when the boys are riding
their bikes to buy comic books, they drive past a house with a late '80s Buick parked in its driveway.
When Francis is purchasing a comic book at the magazine stand, several comics on the rack (The Punisher, Namor, Nocturne & Quasar) are from the 1990s although the film is set in the 1970s.