Turte is missing in one shot when the boys come to the pool party, as mentioned in the audio commentary with Jerry Ferrara and Kevin Dillon.
When Vince, E, Drama and Turtle leave the theater, Turtle leaves to get the car. A little later when the teenage girls scream, he escorts Vince like the others - when the car gets visible: Vince, E and Drama arrive, Turtle is already waiting inside.
When Vince is on the roof of the house, telling all the teenage party-goers how good their lives are, his T-shirt is grossly sweat-stained. Seconds later, it's clean and dry.