When Kevin and Marcus, dressed as Brittany and Tiffany, leave the dress shop with their friends, Brittany's/Marcus' purse is stolen. When he start chasing the burglar down the street, the high-heeled shoes he is wearing switch from no shoes at all to a pair of white running shoes, then back to the original shoes.
When Kevin and Marcus are in the red BMW convertible with their friends, the front seats of the car are seen to have no headrests. However, when Karen turns around to talk to them, the seats are seen to be equipped with headrests. Also, the layout of the dashboard Tori uses to change the radio station is clearly different to the dashboard seen when Karen is talking to Kevin and Marcus.
When Kevin offers the two boys $5 to call him Latrell they are wearing a different set of clothing than when they return to get their money.
When the "girls" are having their sleepover, one of the girls is getting their hair braided. In the first shot we see the head is about 1/5 done, in the very next shot the whole head is done, taking about 5 seconds in real time.
When Tiffany/Marcus is in the fitting room by himself, his skin color around his butt and thigh area is darker as if he needed more make up touch up. Then when Karen enters to help him with the clothes, his butt skin is very light.
When Kevin is driving Latrell's Aston Martin Volante during his date with Denise Porter, the car very noticeably jerks and grinds gears as he pulls up to the house, however, that particular model was not offered with a manual gearbox in the United States.
In the hotel, Kevin Copland (posing as a Wilson) pulls out a New York State license. The license shown is laminated, when in fact true New York State licenses are not.
Many times when the FBI agents remove the Wilson sisters' faces and wigs, their eyes are brown and not blue, even though they would not have had time to change eye color.
In the night club dance-off , when Marcus and Kevin dance, you can sometimes clearly see that it's not them dancing, and in one shot you can see the black hair of the double popping out of the blonde wig.
During the fashion trauma when everyone is fighting: When Russ and "Brittany" were fighting, after Brittany says "My turn" and hits him, causing him to fall into a pile of stacked glasses. You can obviously see that most of them are stuck together, and are fake - judging by the fact that none of them were broken.
In the beginning of the movie when they are in the store, Marcus is behind the counter and he takes a "brush" out of his pocket and brushes his hair with it. It is actually a small mirror.
When they first decide to go under cover as the girls, they show the make up crew painting their bodies. When the second agent goes into the booth, you can see a little bit of flesh colored paint on the band of his underwear, but this is the first time they are being disguised.
In the scene where fictional Brittney Taylor is being auctioned off and she first goes on stage you can see the tags on the dress, one in each sleeve and one sticking out on the back. Also, if she/he bought the dress (as confirmed by the 4,000 charge in a later scene), it wouldn't have tags on, unless Marcus planned to return it, even so, It wouldn't have 3 tags, just one.
At the restaurant, the violins do not move in sync with the music.
Near the beginning of the movie, they are at a convenience store. There is an ad for Excel gum (with slogan "Excel-erate your breath!" Excel gum is only sold in Canada (where the movie was filmed). However, the movie takes place in New York, USA, where they sell Eclipse gum, instead.
It is supposed to be Labor day weekend but there are Fall leaves all over the trees, ground, and road as the girls drive to go shopping.
Given that the FBI was constantly monitoring the Wilson sister's room with a camera outside the door, they would have seen Kevin leaving the room dressed as himself and recognized him.
When Karen, Lisa, and Tori unexpectedly show up at "The Wilson Sisters" hotel room, Kevin and Marcus only have a few minutes to get ready yet their entire bodies are sprayed white again when it took an essentially more amount of time to do so at the beginning of the film.
When Agent Vincent Gomez and Agent Jake Harper are searching the Fake Wilson Sisters' room they find the "Wilson sister masks." They should only find one mask because Marcus is on a date with Latrell Spencer.
When Latrell picks up "Tiffany," you can clearly see his breath as if it's frigid outside, but the movie takes place around Labor Day weekend.
When "Brittany" and "Tiffany" are doing the girls' hair and toes for the sleepover, both the girls hands are clearly not white while the rest of their bodies are.
When Marcus comes home from work he tells Gina it is only
8:00. Gina then says she called the bar, Marcus and Kevin left at 7:45. She also said that MapQuest said it only takes 6 minutes to get from the bar to their house. Marcus says he got there in 8 minutes which leaves 2 minutes unaccounted for. However, if they left at 7:45 then it would be 7:53 when Marcus came home, not 8. That leaves 7 minutes unaccounted for, not two.
In the bathroom scene, Tori says that she doesn't like Heath for many reasons, one of which is the fact that he drives a BMW 3-Series. However, later in the movie Tori is seen driving a BMW 3-Series convertible.
When the agents hold the Wilsons, thinking they are not the real Wilsons, a tattoo is clearly seen on the lower back on one of the sisters, yet neither of the Copelands when disguised as the Wilsons has a tattoo on the lower back.
When the Wilsons are picked up at the airport, the dog is referred to as a "he" when the pet carrier is in need of cleaning. Later, at the nightclub, the disguised Marcus Copeland tells Heath to give "her" to him