When Simon first meets Carol, she sees his scars from being beaten by the men robbing him. As he is telling her the story, the camera changes as he says "I almost died," right before Melvin cuts them off (to finish the small talk in the car). Simon's voice does not match his mouth as he says this.
When Carol takes the train to see Melvyn, after getting out of bed and for the sole purpose of telling him she won't sleep with him, the LED sign on the subway car she is riding says, "Subway Shuttle: Out of Service."
After confronting Simon about his missing dog, Melvin Udall returns to his apartment. The first lock he mistakenly turns 6 times instead of 5, rather than locking it, it remains unlocked. The next couple locks he counts 5. Light switch 5 and counts out loud.
When Simon is in the hotel in Baltimore, he rips his cast between the thumb and index finger. Upon the return to New York, when Carol is exiting the car, the cast is clearly and repeatedly seen to be intact.
When Melvin and Carol walk past the bakery at the end of the
movie, the lights come on and the staff in the bakery can be seen working. Before the lights came on, the staff must have been working in the pitch black.
When Simon spends the night with Melvin, Carol calls and Simon picks up. When it's time for Melvin to talk to her, Simon supposedly sets the phone on the table for him to pick it up. But, when Simon does this, there is a sound of him hanging up the receiver, not setting it aside. When Melvin walks over to it, he picks it off the table as if it had always been there.
When Carol pulls over Melvin's Saab convertible so she can get the full story from Simon the top is up when she pulls over. In the previous scene it was down. There is no way the top could go up at in that short amount of time, and not at the speed she was traveling at, without damaging it. Also, they pull over next to an iron gate with a stone base; although the base of the wall is as high as the car when it pulls over, the gate itself can be clearly seen at eye level during the interior car shots, as can a tree or what might be a stone divider in the gate that is not present as the car pulls over.
When Melvin takes Simon soup, it is past 3am. In the next scene, Carol is up writing Melvin a thank-you letter on the same night; the clock reads 7.20, Spencer is still up, and Carol and her mother are just preparing to go out for the evening.
When Melvin puts on the plastic gloves and is going to pick up Verdell, the glove on his right hand is pulled up to almost his wrist, in the next shot as he's actually picking Verdell up, you can see the glove is only partially on and the fingers of the glove are hanging off his hand.
When Melvin is sitting in the cafe talking to Carol, he removes his glasses and begins to unbutton his breast pocket. In the next shot, his pocket is still buttoned. And in the next, it is unbuttoned as he puts his glasses in.
When Simon, Melvin and Carol go to Baltimore and Carol gets to drive while Simon is telling his story, the convertible has the roof down and its down when she says she'll pull over to give Simon her full attention, but when they pull over the roof is up.
The position of the dog on the rug the first time he is going to eat in Melvin's apartment. First shot, he is on the edge of the carpet, next shot, back on the carpet, and next shot, on the edge of the carpet again. You can tell by looking at the color of the rug.
When Melvin walks to Carol's restaurant for the first time, we see him on the sidewalk, avoiding cracks as he turns a corner. There are spectators in the background, on the sidewalks and in the street, watching the movie being shot; not the normal NYC crowds.
When Carol asks Melvin to take her and Spencer to the hospital in his cab, Melvin shouts at a group of school children. One girl runs to the front with her mouth wide open. She is clearly instructed by the school the teacher to do this.
As Carol gets in the Saab to go to Maryland, the driver's seat belt is caught in the head rest, causing Carol to have to duck underneath. Later in the trip it isn't. The head rest would have to be removed to be able to remove the seat belt, not an easy task on their road trip.