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."
Correction: This is semantics. Keating's being poetic and, yes, while some of the individuals on the wall will still be alive, quite a number of them won't be. Keating is trying to inspire his pupils - going into technical detail about how some of their predecessors are alive and some aren't wouldn't exactly paint the picture that he's trying to create in their minds. He's using artistic licence, if you will, and that's not an error.
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.
Correction: The lipstick is different because you see the other cheek and he didn't draw the same thing on each cheek, although he tried.
Corrected entry: The reading from Thoreau invoked at the meetings is read out of order and misquoted.
Correction: That may be true, but remember that Keating wrote that quote in the poetry book back when he was a student at Welton. He admitted to the kids near the beginning of the movie that back then he was an intellectual "weakling," so it's understandable that the quote is not entirely accurate.
Corrected entry: While the other cast members are dressed in the formal 1950s era clothing expected at a private boarding school during that time, Robin Williams is dressed in casual 1980s chic to include Dockers slacks and suede Hushpuppy shoes.
Correction: This lends itself more to the avant-garde nature of the character. He is unorthodox in many other respects, and there is nothing to indicate his style of dress would be any different. More importantly, the style of clothing that he wears were certainly available in the 1950s.
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.
Correction: It is not wrapped in plastic, it is wrapped in clear cellophane which was invented in 1908 and marketed in the US in the 1920's.
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.
Correction: "Fort Apache," "Red River," "She Wore a Yellow Ribbon," and "Sands of Iwo Jima," and a bunch of lesser movies were all made before 1950.
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.
Correction: Charlie was expelled before the signing for punching Cameron.
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.
Correction: If you look at that scene, it is pretty obvious that the plastic covering the desk set is not hard plastic. It is almost a lamination just used to hold the different components in place. Now, inside the desk set, there are a couple of pins, a pair of scissors, and even a letter opener. If these items were flung from a rooftop at high speed, it isn't all that difficult to believe the sharp objects (especially the scissors.) would rip through the plastic and thus allow the papers to come loose.