Early in the film, Annie and Hallie begin to discover their similarities, one of which is that they both like Oreo cookies with peanut butter. However, Oreos were not widely available in the UK in 1998; they started to become available in packs of four about five years after this film was made. Growing up in England, Annie would probably never have tasted an Oreo.
In the scene when Annie and Hallie fence, they often turn around. This is a red card offense and means that the person who did not turn will get a point and the fencers stop and start again. If the teacher knew the rules for fencing, she would have stopped the match.
When Elizabeth is packing for her trip to meet Nick and switch back the two girls, Hallie comes into Elizabeth's room and tells her that she just talked to Nick and that he's excited to see her. After Elizabeth starts to freak out about the fact that Nick is "excited" to see her, she tells Hallie to go ask/see her "grandmafather." You can clearly tell that she says that and you can tell she realizes she said it because she turns around really quickly.
When Elizabeth and Annie dash out into the rain,and Annie inquires as to what the QE2 is. Elizabeth wrongly informs her telling her its a cruise liner that sailed from London-New York when in actual fact it was used for the transatlantic service from her home port of Southampton, UK, to New York.
In the scene after Nick falls into the pool, there are several shots of Meredith in which both of her hands are visible but there is no engagement ring, despite the fact that she and Nick are at the hotel to plan their wedding, and she asked Elizabeth to make her wedding dress.
During the bedroom scene, where Hallie (as Annie) tells her mother who she really is, she is wearing the "H" locket - which should be in Napa with Annie pretending to be Hallie.
Earlier in the movie we find out that the girls have had the wrong necklaces all their lives. Annie had the H necklace all along, so Hallie-as-Annie would have to wear the H necklace too.
Near the pool, when Annie and Hallie confront their parents and Meredith, Meredith's initial reaction is quickly looking at the twins standing on either side of her. While she looks directly at one, she looks way above the other one's head. Also, in the shots where we see Meredith standing between the two girls, Annie and Hallie seem to be the same distance from camera, but the girl on the right (Hallie) appears to be several inches taller than her twin sister.
In the final scene at the hotel, when Hallie and Annie tell their parents they want to go on the camping trip, the one on the left is missing her right earring. It should have been easy for the audience to tell them apart as the twin who had her ears pierced at camp was shown to have sore ears around the earring, notably in the scene where they're on the phone just before Hallie is discovered.
In the beginning of the switch, it begins with Hallie pretending to be Annie. At the end of the scene she says, "Speaking of dad, I wonder what he is doing right now?" But, a flight from the east coast to London is about 7 and a half hours, plus the time change in London, assuming it was noon in London it would be, four in the morning in California. In other words, Nick Parker would have picked Annie (pretending to be Hallie) up from the airport the day before.
The ships register records the marriage on January 8th, 1986. The switch to Camp Walden is 11 years, and 9 months later. That would be early October, not the typical June-August summer camp time frame.
When Nick and Annie (as Hallie) are driving from the airport to the vineyard, the weather is sunny and the road is dry. As they pass through the entrance into the vineyard, the driveway is very wet and then when they get to the top of the driveway it is dry again.
When Hallie and Annie drag Meredith's air mattress into the lake, they are shown wading several inches into the water in order to push it into the current. However, when they are watching Meredith drift away, their shoes/bottoms of their pants are completely dry.
On their camping trip, Annie and Hallie sneak into Meredith's tent and drag her mattress in to the water. In the close-ups, we see that both girls' hair is half down. But in the next shot, when the girls are dragging Meredith in to the lake, we see one of the twins' hair is down and not half tied up.
When Elizabeth emerges from the hotel before Nick falls into the pool, the sky is bright and cloudless. When Nick climbs out of the pool, the sky in the background is full of clouds, and while he is drying off and talking with Elizabeth, the pool area is darker and the sun is clearly setting. A few moments later, when Meredith happens on the scene, the sky is bright again.
When Annie's mother requests something cold for her forehead inside her hotel room, Annie gets a rag, wets it, and wrings it out. When she hands it over to her mother, it is clean, folded, straight, and dry as if it were just ironed.
After Hallie and Annie have fallen into the watering trough after fencing, they are soaking wet. Hallie's head went completely underwater but when Annie and Hallie remove their helmets, Hallie's hair is completely dry.
After Nick falls in the hotel pool, he gets out to meet Liz. The pool water behind him is rippling as expected. But in the next shot as Liz and Nick are talking together, the pool water is perfectly still, as though nobody had been in the pool for several minutes.
After Nick falls in the pool and discovers the Hallie/Annie switch, he hugs each girl. In the next shot of them, neither is the least bit wet, although their clothes would at least have gotten damp hugging their soaking father.
Numerous problems with hair continuity: When Annie-as-Halley talks to Meredith from the pool, strands of hair around her left ear shift position from shot-to-shot. When Annie tells Chessy who she really is then talks with her dad, her pigtails jump from her front shoulder to her back shoulder three times. When Nick Parker speaks with his ex-wife after falling in the pool, his hair alternates between slicked straight back in close-ups, to combed to his right in two-shots. After speaking with both daughters, his hair is semi-parted and combed toward his left. His wife's hair also alternates from completely pushed back (by a pair of sunglasses) in close-ups to strands hanging in front of her right ear in two shots.
At the end of the opening credit sequence (on the QE2), the photographer comes over to take Nick and Liz's picture. In the shot showing the photographer (from behind the couple), there are lit candles clearly between the couple and the photographer. In the picture taken by the photographer, the candles are absent.
When Hallie and Annie are fencing at Camp one of the girls throws the sword behind her onto the bale of straw, but you see it landing behind the bale, followed by a close up of the sword spinning through the air and landing neatly on top of the straw.
After Nick falls in the pool and then gets out, he meets Liz and they talk. The pool behind Nick (correctly) is ripply. However, in side shots of the two of them, the portion of the pool in the background is completely calm.
The bird that lands on Meredith, waking her when she is on the air mattress in the middle of the lake, is a Red-Crested Turaco, indigenous to central Africa, not northern California where the scene supposedly takes place.
When Hallie says good-bye to her butler, she says she'll see him in 8 weeks. When Annie is driving back home with her father she says camp was 8 weeks long. However, when Hallie is talking to her mother at the dinner table, Elizabeth says camp was 6 weeks long.
When Liz exits the car in the parking lot to the hotel she is not carrying a purse or overcoat. When she enters the hotel she has a raincoat over her shoulder. Then she goes to the front desk to ask about a beige handbag she didn't appear to have before.
Meredith tells Annie (as Hallie) that she is going to marry Nick in two weeks and nothing "Hallie" does can stop it. Later in the movie, they are supposedly at the Stafford and trying to decide if they want to have the wedding there. If the wedding was less than two weeks away, the invitations would have already been sent out with a location on them.
When Meredith meets Elizabeth at the bar, she glances at Elizabeth's bill and says, "You're Elizabeth James?" and then begins talking about how she's designing her wedding dress. Elizabeth James is a common enough name that Meredith would not necessarily have known she was talking to the same Elizabeth James.
In one scene where Annie is sitting quietly, her hair is completely settled and motionless as if there's no breeze at all, yet the propeller on the wooden whirligig in the near-background is spinning rapidly.
When Nick and the girls return from the camping trip, they have a brief conversation with Liz and Nick before heading inside. During this conversation, when Nick is viewed on the left hand side of the screen, he is reflected in the glass of the door they are standing in front of. Although one of the twins is standing right next to Nick, we do not see her reflection, only Nick's. Even when she passes in front of the door, her reflection doesn't appear. (This is most likely a result of the camera tricks used to make Lindsay Lohan appear as twins.)
After their father has fallen into the hotel pool and has emerged from it, Hallie and Annie both give him an intense hug while he is still soaking wet. Nevertheless, in the next shot the clothes of the girls are still perfectly dry.
When the girls are in Meredith's tent during the camping trip, their figures are only silhouettes. One of the girls says something, and the other replies but the sound comes from the same source, proving that in that scene the crew used a lookalike (probably a stunt double) and had Lindsey Lohan say both lines.
In the scene where Annie is looking at Hallie's house, Hallie points to her father in the picture. Hallie's arm noticeably gets shorter, in order to point at the picture. If her arm was unedited, it would point at Annie's wrist.
At night, when the girls leave their tent to drag Meredith to the lake, the leaves of the trees behind the tent are clearly visible through the open tent door indicating that the tent is open on the other side as if there was nothing there.