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.
When Zach comes out of the shower with a towel around his waist and stands at the mirror of the medicine cabinet in a side shot, his shoulders and upper body are visibly wet. When the shot briefly changes to a full back shot of him, his back is completely dry. When it reverts to the original side shot, his body is wet again.
(at around 14 mins) When Zach and Shaun first converse, Zach, in a medium close shot, asks Shaun, "You want to go?" and his mouth is then closed. A immediately following edit to a more distant shot shows him smiling broadly with his teeth showing.
The wetness of Zach's and Shaun's hair during both conversations on the beach, esp. in the scene with Gabe, after they went surfing. The hair appears dryer, then wetter again, as if they poured water on the two actors between takes again.
During the break-up scene with Shaun and Zach in the car, a side shot shows a tear running down from Zach's right eye leaving a track on his face. In the following full front shot, there is no trace of the tear track. When it reverts back to a side shot, the tear track is visible again.