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.
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 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.
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 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 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.
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 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 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.
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.
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.
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' 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)
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.