When Will first meets Sa5m in the high school cafeteria, Sa5m writes her name on a notebook of lined paper. A close up shows plain paper. The next frame is again showing lined paper.
When Charlotte hands Will her notebook in Wills bedroom, and we first see 'Phil's Song', the words 'Phil's Song' are written in red writing. But when Charlotte plays 'Phil's Song' on the piano and we see the notebook again, and we clearly see that the writing of 'Phil's Song' is now in newspaper cuttings, not red writing like it was previously in Wills bedroom.
In the opening shots when he's making his "It's Monday Morning" entry to David Bowie, there's a closeup of his hands on the keyboard and his watch reads, 1:30.
When Charlotte comes to talk to Will about the Burning Hotels sneak peek, Will is listening to his iPod and has his earbuds in, until he takes his right one out. In the next shot, both his earbuds are out.
At 7:12, the man on the right side with the white striped shirt appears briefly in the background, but when the camera cuts away and comes back, he is gone at 7:43. But then he comes back at 7:49.
The shot of Sa5m's name in Will's hand is on unlined paper because they were filmed at different times. Will's hands belonged to a stand in who was on set after both Vanessa and Gaelan had returned to their trailers.
As Lisa Kudrow leaves the bathroom after 'talking' with Will, then leaving you can see her move of the way. Presumably to get out of the way of the camera.
When Charlotte inserts a CD copy of the self-titled 1966 debut album by The Velvet Underground and Nico into her car radio to teach Will the basics, it begins playing "Femme Fatale". "Femme Fatale" is the third track on that album, while the first is "Sunday Morning". "Sunday Morning" should have started playing, not "Femme Fatale".
When Will is pretending to tie his shoe to avoid a run-in with Ben, he clearly appears to press pause on the click wheel of his iPod. Therefore, the music heard on his earbuds would have cut off immediately. However, the music is still heard playing until the exact moment he takes out one of his earbuds in the next shot, at which point it cuts off.