When David hits himself in the chest repeatedly after he talks with Mr. Walker, Nicole comes home and tells her dad that she saw bruises all over his chest, but shortly after David carves "Nicole 4 Eva" on his chest, there are no bruises or any marks.
When Margo comes to visit Nicole at her home it is dark outside. When the two girls go outside a few minutes later it is light.
When Steve comes home and angrily finds David in his backyard, he watches David throw Nicole into the pool. Moments later, David is kissing Nicole's forehead and her hair is dry.
We see and hear Margo being brutally beaten and punched in the face. But a short time later, she only has a swollen cheek.
When Nicole and Gary talk in the school cafeteria, her hair jumps around several times between shots.
The coded lock for the front door is mechanical, not electrical; therefore the beeping is fake, and the lock is by no means attached to the security system. Moreover, if the alarm system had been disconnected, the entry code would be useless for getting inside.
The note that Nicole writes, and the notes that David writes, all have the same handwriting. It's evident from this that the same person wrote all these, be it David or Nicole or neither.
When David carves the tattoo "Nicole 4 Eva" on his chest, we see him use a broken piece of pen to carve the letters. A moment later he hoses off the blood and then wipes down his chest. Not only would the blood continue to run out of the cuts immediately, but there are no signs of any cuts on his chest. Its obvious the words were simply written on.
We see David kick Gary viciously & repeatedly in the ribs. Yet the next time we see him, which is a weekend away at most, he is walking with no discomfort whatsoever.
When Toby is in the Wagoneer dialing 911, we see the numbers appear on the display but the background does not light up. Its obvious he is not really dialing the phone.
When the power is cut to the Walker house, we see Steve continue trying to use the cordless phone. A beeping is heard as he presses the buttons; but if the power were gone, there would be no sound.
When Nicole and Margo first walk into the rave, they pass by a young man with a ponytail who wears an orange shirt; he smiles at them. Moments later, they pass by the same exact man again.
The popularity of Rebelde (2004) - and of Britney Spears "Baby One More Time" video - notwithstanding, it is very unlikely that students at any public school would be permitted to wear attire as revealing as Margo's and Nicole's. Also, it is even less likely that 20-somethings (like David and Company) would be allowed on and off school property without a reservation and/or a security check.
Laura's hands change positions much too fast, between peeling shrimp and resting on the counter, after Steve slams the cabinet door angrily.
When David and his friends are about to invade the Walker house and Laura is opening the door, we hear David say to his friend "Terry, go out there and keep a watch." But his mouth says something different.
When David and his friends go to Margo's house so that David can apologize to Nicole, she attempts to leave in her SUV. When the camera is focused on the back of the vehicle, you can see the reverse lights on but can also hear the engine turning over at the same time. The reverse lights only come on when the vehicle is in reverse and also cannot be started unless it's in park or neutral.
When Margo comes into the house and says they found Gary dead; she says that he was beaten to death - however, David snapped his neck.
When Nicole and Margo are in the bar for the first time, we hear and see Nicole ask Margo "What do you want to eat?" and then we see her instantly ask "What do you want to eat?" a second time.