Continuity mistake: At the beginning of the first episode of Season 3, "The Aftermath", Ryan's hair is much shorter than it was at the last episode of season 2. Since they arrive at the hospital with a bleeding Trey, we know that this is right after Trey got shot. It doesnt make any sense that Ryan would go get a haircut right after Trey got shot and then go to the hospital with him.
Continuity mistake: At the beginning of the second episode of season 3, "The Shape of Things to Come," Summer, Marissa, Ryan and Seth are ordering pancakes at the restaurant. In this whole scene, Marissa's blue mug keeps changing position between shots.