Plot hole: When Evan is in jail with the religious prisoner trying to get him to help him get his journals back he goes to the scene where he is drawing that homicidal picture in kindergarten, but he gets up and puts the spikes that holds documents through his hands, creating a stigmata-style scar. The religious guy in the cell with him is so amazed because of this he thinks Evan is a prophet and he decides to help him. If Evan had gone back in time and got those scars on his hands, he would have changed the original timeline and would have arrived in jail with those scars the whole time. Some people try to correct this using the "If I can create scars, then can I fix them?" statement Evan made to defend the mistake and suggest he can create instant scars but he was using the word "scars" to refer to the negative events; not literal scars on his body. The scar he got when he burned himself in the past didn’t magically appear on him the moment he returned from the past; it became part of a new, slightly altered timeline (just like the scars on his hands should have been) and it let him know he can change history.
Continuity mistake: In the final shot of the hypnosis scene we see Evan from above. The blood on his shirt that was visible in the last shot has vanished.
Continuity mistake: After little Kayleigh picks up the blockbuster, older Evan is now at Sunnyvale Institution. The blankets are not tucked under the mattress, then they are tucked when he gets out of bed, and in the shot from under the bed, as he searches for his journals, the blankets hang down to the floor, yet in the next shot they are not.
Continuity mistake: After Evan beats Tommy to death on the college campus, it shows him kneeling over the body, and there is no blood on his collar. The shot changes to Kayleigh screaming, then back to Evan, in the same position, but now with blood splatter on his collar. He had already finished assaulting Tommy, so there shouldn't have been anymore blood spray.
Continuity mistake: When Ashton Kutcher convinces his jail bunkmate that he is personally in touch with Jesus, by using his journal to cause stigmata to appear on his hands, a stigmata mark can be seen in his right palm before he changes the past (just as he mumbles "tell me if you see anything weird").
Continuity mistake: When Evan wakes up in a mental ward, after he killed Kaylee, the camera zooms around him in bed, which is in the middle of the room. When he gets up to get his journals, the bed is on the left side of the room against the wall.
Revealing mistake: Although the film is supposedly set in New York, there is a Canadian speed limit sign in one of the first shots. Across the street which Evan lives on.
Plot hole: In the scene where Evan is in Sunnyvale institute and demands his journals, the doctor says he has never had any. The very same doctor told him to start writing in journals when he was seven before he killed Kaylee, so when Evan went back in time to destroy the firework, he couldn't have erased journals that were written before that. Evan was only allowed to write 2 pages before the incident where he killed Kaylee. This event completely altered his history, allowing the courts to place him in the mental hospital afterwards. There was no continuation of writing the journals because in this timeline his blackouts were not the main issue.
Other mistake: When Evan finishes talking to the "prostitute Kayleigh" on the restaurant, he goes to his room, which finds his friend having sex with a girl. When Evan demands them to leave the room, he sits on the chair, and opens his diary, but you can see that when he opens it, the pages are empty. He doesn't turn to another page before he begins to read, so he appears to be reading from the blank page.
Plot hole: When the frat boy bursts into Evan and Kayleigh's romantic dinner to tell him his car's been trashed, he says he's been looking all over for Evan. But he was sitting right there watching the whole time Even was giving the pledges their instructions for helping him set up the romantic dinner (which necessarily included the exact time and place), so shouldn't he have known where Evan would be?
Continuity mistake: When Evan goes back in time to when they are putting the blockbuster in the mail box, his cigarette falls out of his mouth and onto his shirt. He gets burnt, as is shown when he returns to modern times, but all the rest of the scenes with them in the forest just after it happens, there is no burn mark on his shirt.
Continuity mistake: When older, armless Evan is sitting in his wheelchair reading the journal entry that says, "Today Mommy is taking me to play with Kayleigh and Tommy." the writing appears to be on the bottom of the journal page. Yet when he goes back to that time, the writing is at the top of the page just as it was in the original scene earlier in the movie.
Continuity mistake: When Evan gets his first nosebleed, during the hypnosis session, the blood on his face changes quite a bit as shots change while the doctor is trying to wake him up. One shot shows two streams of blood, another shows only one, and yet another shows hardly any blood at all.
Continuity mistake: In the scene right after Evan visits Lenny for the first time after 7 years, he is lying on his dorm bed, flipping through his old notebooks. The scene starts out with him biting the nails on his right hand, but in the next shot when the notebook starts shaking, both hands are visibly holding the notebook.
Continuity mistake: When Evan takes Kayleigh (who is now a drugged up hooker) to a diner, there is a plate with a piece of pie, a wallet and a red cigarette lighter on the table in front of Kayleigh. Each time the camera turns to Evan and then back to Kayleigh, the lighter is in a different location. This happens several times.
Audio problem: When Kayleigh, Tommy, Evan and Lenny are putting the block buster in the mailbox, Evan puts the cigarette on the block buster and says it should last Lenny 2 minutes, but his mouth says 10 minutes.
Visible crew/equipment: In the last scene, when Evan is walking down a busy Manhattan street, talking on the cellphone, you can see the red camera truck's reflection in a window on the other side of the sidewalk.
Revealing mistake: In the first scene of the film, Evan is writing frantically in a journal and his last words that the camera zooms in on are "Save her". Look closely and you can see that those same words are already indented on the page as indicated by a visible letter "s" and "h" and a period that had already been written on the pad of paper. Obviously they had done some other takes.