Continuity mistake: When Evan tries to get his journals back from the bullies in the jail, the object that the guy on the bed is reading alternates from a closed copy of Hustler, to a journal and back several times. (01:18:35 - 01:19:20)
Continuity mistake: When Evan and his mom go to the psychic for a reading, the position of the money in the glass bowl differs depending on camera angle. (Director's cut) (00:40:30)
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"). (01:17:05)
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: At Sunnyvale, when Evan bursts into Dr. Redfield's office the Doctor stands up and in the wide shot Dr. Redfield starts to put his books down on the desk, but the way he holds the books differs in the next close-up.
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.
Continuity mistake: Now a resident at Sunnyvale, when Evan looks under his bed for his journals, in the shot from under the mattress the blood stains that appear on his white tee differ from the stain in the shots before and after.
Continuity mistake: As the teacher and mom walk by the desk, one of the pink papers is not fully impaled on the spike, but a moment later it is. (00:03:45)
Continuity mistake: When Evan and his mom are at the restaurant together the flowers, food, etc., change depending on camera angle. (00:38:45)
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. (01:09:25)
Continuity mistake: When Evan shows up at Kayleigh's apartment, in the close-up she covers the plate with the newspaper. In the next shot from the doorway the newspaper, bottled water, etc., are positioned very differently. (01:29:35)
Continuity mistake: When armless Evan goes back, the journal, cereal bowl and OJ are visible in a close-up and in the next shot young Evan rushes away from the table. The position of the journal is quite different in the two shots. (01:44:45)
Continuity mistake: As Evan eats the burger at the Hilltop Cafe, the bit of burger left in his hand after his last bite differs in the next shot as he puts it down. (01:27:25)
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. (00:05:30 - 01:44:35)
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. (00:18:00)
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 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 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.
Continuity mistake: When Evan has a nosebleed after finding out that he has no hands, there is blood on his neck that disappears and reappears between shots.
Answer: The point of the movie was that, no matter what he did and how he tried to change things, they always ended up bad. If he went back and did that, something unforseen would have happened to make things terrible. Evan realized that everything bad that happened to them was because of him. He then decided the only safe way to make things right is if he just took himself out of their lives all together. That's the logic the filmmakers went by, if you don't want to accept that, then you will just have to consider it a plot hole.