Errors made by characters (possibly deliberate errors by the filmmakers): Tom claims to have invented the paper cup sleeve when in reality it was invented by a man in California named Jim Chelossi.
Revealing mistakes: Opening Scenes: when showing the Brooklyn Bridge/Manhattan Bridge with Verizon building in the background, the picture is flipped. The Brooklyn bridge is SOUTH of the Manhattan bridge when looking at Manhattan. You can barely see the Verizon logo spelled backwards.
Factual errors: In the scene where Tom calls Hannah, she says it is 3am in Scotland. With the 5 hour time difference, that makes it 10pm in New York, yet light is shining through Tom's window.
Anachronisms: In the opening party sequence, a logo sheet can be seen hanging on Hannah's dorm room door displaying various Cornell logos and insignia, including the redesigned crest logo, which wasn't in use until 2004. In 1998, the official Cornell logo was still the oft-mocked "Big Red Box", which was frequently criticized as looking too much like a clone of the square J.C. Penney logo.
Continuity: In the scene where Tom and his friend are looking in the window of the bridal shop, and Tom sees his reflection in the window with him in the bridesmaid dress, it shows Tom having bare arms and neck so the dress allusion is complete. In the movie, however, he is wearing a polo and the sleeves longer than those of the dress, and the collar would show above the collar of the dress.
Continuity: Near the end of the movie Tom is being driven to the airport. A few seconds later they cut to Hannah on a ferry boat. The Land Rover in both shots has the same license plate.
Continuity: In the second scene when Tom is driving fast through the streets he cuts off what seems to be a white police car, but in the next shot behind him is a yellow cab and it switches to a van in the next shot.