Deliberate mistake: When Matt is spying through the telescope on Casey's argument with her parents, you can hear Casey yelling "No no no" and hear her bedroom door slam, etc. This is all from Matt's point of view from the telescope - it would have been impossible for him to hear any of that, being such a far distance all the way across the lake.
Deliberate mistake: In one of the scenes where Johnny Depp goes into the bathroom (specifically, when he changes clothes with the other man), Christopher Walken's character looks under the stall. The camera then shows the legs and crotch of a man sitting on the toilet, who still has his boxers fully on, but his pants are down.
Suggested correction: That man was not actually using the toilet. He was posing as Watson (Depp) so that Walken would think he was in the stall the whole time.