When Mary and Pat are at the carnival, her cotton candy shrinks faster than she is eating it.
After Norm gets out of his car to deliver his pizza you see Pat drive from behind the house. It seems odd that Pat is in his car behind the house before Norm even gets there.
When Ted and Mary take Warren fishing, Ted gets a fishing lure accidentally hooked through his mouth, with hooks piercing his lip and cheek. However, in the very next scene (and for the rest of the film) he has no bandage, no wounds and no scars.
When Ted first arrives at Mary's house for prom, Warren is seen in the background with the Rubik's cube. He has just finished solving it, and holds it up slightly. In the next shot of Warren, he is again finishing solving the Cube and again holds it up.
When Ted greets Mary in Miami for the first time Mary and Warren are standing on some steps. A man in a suit behind them goes down the steps twice.
When Healy is trying to revive the dog, he pulls two wires out of the table lamp and taps them together to make sure there is power. Letting hot and neutral wires touch as Healy did would cause a short-circuit and blow the fuse or trip the main breaker, potentially shutting everything in the living room off, and preventing Healy from zapping the dog.
Ted is given Pat's address in Florida. But when Ted sneaks in before Pat and Norm, Pat acts as if the apartment is not his but Norm acts as if the apartment is his.
When Ted is at the chiropractor after helping the older man move, the doctor says tenderness left of "L-7". There is no L-7. Parts of the vertebrae are labeled: C-1 to C-7, T-1 to T-12, and L-1 to L-5. However, the term "L7" is a slang term meaning "square", and this is most likely a subtle joke at the expense of Ted.
Tucker asks Pat if he designed the Estadio Olímpico in Santiago de Chile. Although there are six different soccer stadiums in Santiago, not a single one is called "Estadio Olímpico". Since Tucker was a fake, hadn't actually been to Chile, and sensed that Pat was a fake, he probably made it up to see what Pat's reaction would be.
When Ted goes into Pat's apartment, there are several piles of dog poop on the landing by the door. When he leaves, the audience can only see one pile.
While sitting on the car with Ted, Mary looks at her watch to see what the time is but the watch face is facing us, not her.
When Ted is caught is his zipper, his jacket appears and disappears between shots.
In the shot where Ted ducks to avoid being mauled by Puffy, a hand can be seen on the left side of the shot throwing the prop Puffy out the window.
After Ted is attacked by the dog in Mary's apartment and slides across the floor, he attempts to crawl away from the dog. The audience then see the dog bite Ted's ankle and the next shot is Ted's fingernails scratching the hardwood floor as the dog drags him back. If you pay close attention, Ted's fingers slide past the scratches made by the crew before the shot ends.
When Warren is attacking Ted because of the baseball, you can see Mary messing with her strap.
At the end of the movie, Magda's boyfriend shoots Jonathan, the singer. When Jonathan falls backward into the water behind him, the audience hears a splash. However, the audience does not see the water rise from where he falls in. Also, there are no ripples in the water.
Dom says each day is better than the next instead of last.
In one of the flashbacks to 1985, as Mary and Ted stroll down the street, she rants about San Francisco 49'ers' quarterback Joe Montana being obscure and underrated, but insists he will surprise by taking his team to a SuperBowl. By 1985, Montana had already won two Superbowls with the 49'ers.
The Chevy Caprice station wagon Ted was driving in 1985 had "European Style" headlamps. Chevy Caprices did not have "European Style" headlamps until 1987.
The "1985" flashback at the beginning of the film was most likely originally set in 1980 or 1979. The wardrobe, hair and cars in these scenes are all consistent with the late 70's. In addition, Mary's comments about "unknown" Joe Montana would make more sense, in that Montana was drafted in 1979.
The film features two troubadours several times throughout the film. Though the audience can hear the vocals and accompanying guitar and drum lines, this music almost never matches what the drummer and guitarist are actually doing with their instruments on screen. This was intentional on the part of the filmmakers and meant to be humorous.
After Ted's "interview' with the detectives, the audience sees Pat spying on Mary as she returns to her office. In two back-to-back shots the shadow of the camera operator is cast (and moving) over the black convertible - lower right corner of the shot.
A crew member's hand can be seen on the left side of the shot throwing the prop Puffy out the window.
A running gag throughout the film is Warren's sensitivity to his ears (he freaks out when anyone touches them). However, in the 1985 flashback where a guy was setting him up for a prank, he didn't react when the guy whispered in his ear.
Mary tells Ted that Puffy is "A border terrier, like Benji." But Benji was not a Border Terrier. He was a mongrel, mixed of Miniature Poodle, Cocker Spaniel, and Schnauzer breeds.
When Ted sees Mary in slow motion at the beginning of the movie she is riding her bike. Then after Ted stop the fight between Warren and the other guy, they walk back to Warren and Mary's house. So Mary just left her bike at school.
Mary did not think of tying Puffy to the roof of her car. It is very dangerous to drive a vehicle with a loose attachment on the roof.