Continuity mistake: When Alex and Alison get out of the limousine at the premiere, Alison drops her purse and Alex bends down to pick it up, splitting his trousers. The purse is in his right hand, the angle changes and the purse is now in his left hand, then another angle change and it's back in his right.
Revealing mistake: Alex tries to commit suicide in the swimming pool. Spencer and Baz rescue him by lifting him to the water's surface and floating the unconscious Alex across the water's surface. Just before he reaches the side of the pool the unconscious Alex reaches out and touches the side wall of the pool.