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.
Steve Walker is on the phone in his office. In one frame he is holding the phone in his left hand. In the next frame he is cradling it with his shoulder, then reaches up with his left hand to hang it up. It is also momentarily a cordless model.
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.
During the fun park scene, while David and Nicole are kissing, her left hand is caressing his cheek and her nails aren't manicured. The next scene, on the roller coaster (pulling up the bar), all of her nails are done with tips. Then, when David is touching her thighs, the manicure is gone.
Nicole's hair keeps changing from neat to messy and back again at several points in the movie: near the beginning, when she and Margo and Gary are in the bar; when she sits on the park fence with David, shortly after the police break up the rave at which both youngsters hook up; when she comes to surprise David at his party and finds him with Margo; near the end, when she sits on her bed while David explains why he'll have to kill her father.
Nicole's figure fluctuates noticeably, if slightly, throughout the movie...particularly when Margo comes over to see her, and they argue about David's being less than faithful to Nicole. As Nicole shows up wearing a purple blouse and red jeans, she is gym-trim. When she storms out after Margo just seconds later, Nicole is somewhat heavier - almost curvaceous. Since the entire film takes place over a couple of weeks at most, it is unlikely that Nicole simply went on a diet.
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 popularity of Rebelde - 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.
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.