Revealing mistake: When Kate decides to ditch Maguire and escape herself, we get a shot from above showing her face and Maguire's feet. However we only see his trousers. His feet are gone. Even though some could say that the wall removed them, but we can also see that the wall hasn't reached the part of the feet that are missing. It appears the CGI team forgot to add his feet in.
Revealing mistake: When Kate opens one of the doors to find Simon on the other side ("Hello gorgeous, miss me? Huh? Come on in, the water's fine."), the shot of her climbing up to open the door is a recycled shot from 42 seconds earlier. It's pretty obvious as the shot still contains a mistake from when the shot was used previously - a boom mic is reflected in the door's shiny surface as Kate climbs up to open it.
Revealing mistake: The way Jerry comes into play is problematic. He is shown coming up from the bottom of the room where Kate has just met Sasha. He appears to be climbing upwards, which suggests that the gravity in the room that he is leaving is the same as the one that he is entering. However, the way that he positions his body around the square opening doesn't make sense. He turns around to see the girls. At this point, it would be very difficult for him to stay up, because he no longer has support like he did when he first came up. This shows that the actor was standing on something.