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 pitch black.
When Carol and Melvin are talking in the restaurant before the trip to Baltimore, Carol folds up the pages of the "Thank you" letter but they are immediately unfolded again.
When Carol, having been caught in the rain on her way to see Melvin, enters Melvin's building, her bra straps are visible through her wet clothing. When she reaches Melvin's door, her wet shirt shows that she is not wearing a bra.
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 Simon picks up his dog from Melvin after returning from the hospital: he's at Melvin's apartment door in a wheelchair but from previous scenes we know there are stairs between the apartments.
When Melvin picks up Simon's dog and it urinates, you can see that the stream of urine is clearly coming out of Melvin's gloved index finger.
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.
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.
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.
Carol calls Melvin before the trip and asks him if she should pack something to dress up. Melvin is packing (with a checklist) and has a lot of stuff laid out on his bed and says yes. Yet when they get there and go to a fancy restaurant he has to buy a suit jacket and tie because he didn't bring either one.
At 2:32, Melvin turns on the kitchen light then he opens the refrigerator door. The refrigerator interior light does not turn on. It is dark when Melvin grabs a bottle of water.
When Melvin visits Simon late at night (around 64 minutes in) both men appear momentarily distracted by Simon's front door banging.
Carol was wearing a bra when she went to see Melvin and got caught in the rain, but she got so thoroughly soaked that it didn't look like it from the front. This is why she was surprised when she looked down. Nothing was being hidden from Melvin's view despite having the proper undergarment.
When Carol comes to talk to Melvin at his apartment in the middle of the night when it's raining, just after she turns towards the camera during the long pause in their conversation when both of them are staring at one another, a drop of water can be seen briefly falling from the ceiling on the right hand side of the shot. It is raining outside, but there is no indication of a leak in Melvin's apartment.
When Carol tells Melvin that he can bring the dog into the restaurant, her mouth isn't moving.
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 Carl pulls his car over to the curb where the guys are standing he tells his friend over the speaker-phone "OK, I just found a model", but his mouth does not move.
When Melvin and Simon both coax Verdell to come to them, Simon holds out the bacon and says in baby talk, "Oop" twice. But the second time he says it, his mouth does not move.
When Carol is waving goodbye to her son, there is a reflection of camera equipment clearly visible in the window.
During the scene with Melvin on the pay phone, a sign for the Holland Tunnel is visible when he, Carol and Simon should in fact be on their way back to New York.
When Carol and her mother are at the kitchen table.
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.
Despite his OCD leading to forced hygiene, Melvin touches several door knobs with his bare hands during his visit to the psychiatrist.