Continuity mistake: When Ryan is lighting Marissa's cigarette with his, at the start of the shot it's full, then the shot goes to Ryan and it's still full. But then the shot goes to Marissa and if you look at Ryan's cigarette you can see it is almost all gone. Then the shot goes to Ryan and the cigarette is full again.
Continuity mistake: When Marissa is dumped on the driveway by Summer and Holly, Ryan first sees her there her left arm is across her chest with her elbow sticking up. The next shot of Marissa her ams are straight and spread out, and the next they are close by her sides, which is impossible for her to have moved them herself as she is unconscious.
Other mistake: After Kirsten is talking to Sandy about the fact Seth got into a fight and she comes inside to see that Ryan has made breakfast, there is no way he could of set the table, found all the utensils and ingredients and cooked them by the time Kirsten gets back inside.