When Anna plays guitar, some of the chords she makes are different than what's being heard.
When Tess (in Anna's body) wakes up after the swap, she makes her way out of Anna's room and the door, which was removed the night before, is still there. In subsequent shots, and conversations, it's obvious that the door isn't supposed to be there.
When Tess is in Anna's body she finds out that Anna has pierced her navel, but a short while earlier, Tess tells Anna to pull down her shirt and her navel isn't pierced.
At the rehearsal dinner, when Tess and Anna switch back, you can hear Grandpa shout "Earthquake!" but in the camera shot you see him looking around and his lips are not moving.
When Anna (Tess) needs a ride home and asks Jake if he take her, the camera is reflected in Jake's helmet.
When Tess (in Anna's body) enters her room with Harry, the shadow of the camera falls on the door.
When Tess (in Anna's body) is asking Jake for a ride home he tells her no because she wasn't the girl he thought she was. He then mentions what she did to Stacey Hinkhouse's test. Tess says that she didn't even know what she was doing and she didn't mean to do that, but her lips aren't moving.
When Anna (Tess) and Jake are talking in front of the house before they leave to go to the wedding rehearsal, the highlights in her hair change positions while they are talking back and forth.
During the scenes where Anna (Tess) is at school, the part in her hair appears and disappears between shots.
When Tess (in Anna's body) asks Jake for a ride home, the boom mic is reflected in Jake's glass shield of his helmet.
When Anna (Tess) tackles Jake outside the house when he is singing he falls sideways, but in the next scene Jake appears to be standing up and Anna (Tess) is holding his stomach instead of his side.
When Ryan is dropping Tess (Anna) off at work and leans over to kiss her, his tie is hanging over his arm, since the seat belt caught it, but in the next shot it's hanging straight.
When Tess is getting out of the taxi Harry comes running up with the underwear on over his eyes after trying to get it off Tess sends him back. Harry's right eye is visible for a moment then before he hits the door it is covered back up again.
In all but the third close shot of the drummer at the House of Blues auditions (while he mouths at Anna), the drums you hear are different from those he plays (you hear cymbals when he never actually strikes any, etc.). In the third shot, the audio and video appear to be synchronized.
The new piercing in Anna's (in Tess' body) ear shifts position slightly throughout the movie.
During the House of Blues audition, in the intro, you see the backup guitarist with the black stratocaster playing. Since Tess (in Anna's body) isn't playing, palm mutes are heard. It looks as if the backup guitarist is playing the palm mutes, but her palm is nowhere near the bridge or on the strings. She's strumming the guitar, and Anna originally plays the palm mutes.
When Jake is standing outside the house, singing, Tess (in Anna's body) comes racing down the sidewalk to knock him down. Yet, after he falls, he has fallen on grass. He should have fallen on the sidewalk.
The school scenes were shot at Palisades Charter High School in California. The crew renamed the school to "Sunset Ridge High School", but in the gym scenes, the Pali logo (a "P") is clearly seen in the middle of the court.
When Anna returns home from school on the Thursday, she finds Harry banging on her guitar with drumsticks in her room upstairs. She chases Harry downstairs and goes straight into the garage and picks her guitar which suddenly appeared downstairs.
When Anna gets home from school and finds her brother in her room her hair is all ruffed up. When she goes into the garage to practice her hair is smooth and sleek, then when they are finished her hair is all ruffed up again.
When Tess (in Anna's body) is walking she is carrying two bags. Her Palm beeps and she drops her bags which are empty.
When Anna (in Tess' body) and Tess (in Anna's body) are in the car. You can clearly see the outside of the window is fake. You see that the same sign keeps appearing over and over again (on "Anna's" side)
On Thursday night at the Chinese Restaurant, Anna is wearing nail polish (most noticeable when she reads the fortune cookie note). When we see her in bed on Thursday night, the nail polish is gone. However, on Friday morning she's wearing nail polish again, and it appears to be a different shade than she was wearing at the Chinese Restaurant.
When the band is practicing in the garage, Anna is jumping up and down just before her guitar solo. In the very next shot, when her solo begins, she is solidly on the ground and steps forward to play.
On Friday morning, Harry puts the cereal box down with the front facing himself. In the next shot, it is facing the other way.
When Anna (Tess) goes in to wake Tess (Anna) after realizing that they've switched bodies, there is a turtle hanging on the wall. When Anna (Tess) walks around the bed to grab Tess' (Anna's) feet, the turtle is no longer there, but is again there when they run into each other to try and switch back.
When Anna (Tess) goes in to wake Tess (Anna) after realizing that they've switched bodies, she walks around the bed to grab Tess's feet and pull them. She falls to the floor and you can see her bare foot. You can see she is wearing socks in another shot after that.
When Pink Slip is playing "Ultimate", the same group of people near the stage appears and disappears throughout the song. One girl in the group reappears in the crowd sometimes.
Anna gets a cartilage piercing while in Tess' body. Later that day Tess, while in Anna's body, instructs her to remove it. The piercing hole would have closed. However at the wedding reception the piercing is back. It would be assumed that she would not get it pierced being back in her own body.
Tess has Anna's door removed, next morning Anna and Tess "Switch" and stop half-way down the stairs. Behind them Anna's door with her sign "Parental Advisory Stay Out Of My Room!" is still there.
When asked, Anna tells Jake that she plays her "dad's old Telecaster". The guitar she plays in the movie is not a Telecaster but a Telesonic that wasn't produced and sold until July of 1998. However, the Telesonic is a guitar made by Fender themselves and based heavily on the design of the Telecaster. It is considered a 'supersonic' version of the original guitar, hence the name 'Telesonic'. It would be quite reasonable, for the sake of keeping explanation to a minimum, for Anna to refer to her guitar as a Telecaster.
When Anna (in Tess's body) was woken up by Tess (in Anna's body), Anna tried to fix her hair as she got up but Anna was fixing her hair like she was still Tess - brushing the ends of her short hair. As "Anna", she should have instead tried touching her "long" hair.
When Tess (in Anna's body) is trying to talk with Stacy before class the teacher Mrs. Brown says "alright everybody - let's take our seats for the test". Stacy then tells Anna "sit with me?" however students wouldn't be allowed to pick and choose their own seating arrangements especially when a test was being given.
When the band PINK SLIP is playing at the House of Blues the name of their band is on the drums but not when they play at the wedding reception.
When Ryan is driving to school and reaches for Tess's hands, you can tell that his Navigator is in Park although he is supposed to be driving.