The night after Zach and Shaun kiss for the first time, Zach is shown to wear a different shirt when he drives back home. However, he wears the shirt he had worn in the previous scene when he reaches home.
When Shaun walks past Zach in the back of Zach's truck ("Just leave it, dude"), Shaun is carrying his long board. When he is running to the surf and first meets Cody, he is carrying a different, shorter board. This is only time in the film that Shaun uses anything other than a long board.
In the scene after Gabe, Zach and Shaun finish surfing Gabe says that he's never going on a long board. When he says the line and the camera angle changes to Zach and Shaun, in the background you see people on the cliff, one has a red sweater on. When the scene changes back to Gabe and he says he's going to get some beer and leaves, the people are no longer on the cliff when the camera angle switches. (Trevor also mentions that fact in the commentary.)
Were they using two different GMC Jimmys? In the first scene where Zach drives up Gabe/Shaun's driveway it looks/sounds like he put the truck in Park, but in subsequent scenes in the truck he was shifting a manual.