Corrected entry: In the beginning scene where Mr. Keating is giving his initial "carpe diem" speech in front of the trophy case, he mentions something to the effect that these students "are now fertilizing daffodils." In 1950, while some of the students would be almost 50 years out of Welton, some of those students would be only 20 or 30 years removed from Welton, and nowhere near the age to be "fertilizing daffodils."
Corrected entry: When the Dead Poets are having a meeting in the cave and Charlie brings the two girls, he asks to be called Nowanda and draws a symbol on his cheeks in lipstick. The scene changes to Knox at Chris's party (concurrently happening) and then back to the cave. The second scene in the cave is supposed to be directly after the first one, and the lipstick on Charlie's face is different.
Corrected entry: In the scene where Todd and Neil are on the roof and Todd is lamenting the fact that his parents once again gave him a desk set for his birthday, the desk set shown was wrapped using modern plastic packaging techniques that did not exist in the 1950's. Such a desk set would have probably come unwrapped in a box.
Corrected entry: At the beginning of the movie when the boys first meet Mr. Keating, he does an imitation of John Wayne. In the 1950s that would have been an obscure reference as Wayne was not that well known an actor, even though he started appearing in films in the 1930s. His fame exploded in the 1960s with all of his western and war movies.
Corrected entry: At the end of the film, when the boys have all signed the document except one, they show the last one signing his name on the document, and there isn't a signature for each character that should have signed it. One signature/character is missing.
Corrected entry: When the 2 guys are on the roof and they are talking about the desk set one of them just got for his birthday, it is covered in plastic. However, when they throw it off the roof, papers and pens fly everywhere.