Other mistake: When Percy and Simon are driving to the hotel and it starts to rain, the rain is dripping downwards on the windshield. Normally when a car is driving the rain should be moving up (because of the wind).
In the scene where Simon is having dinner with Theresa's family, Theresa has a bowl of salad. She puts some salad on her plate and then some on Simon's plate. When the shot changes she is putting salad on her plate and Simon's plate again.