Continuity mistake: When April shoots the alien, they all go outside to look at it. Behind them, you can see the windows boarded up, but they don't board them up until later in the movie.
Continuity mistake: When the kids are looking at the Stonehenge-type structures, the younger boy calls for everybody to come and see the hidden message. From the shot in front of the boy you can tell from his body position he's sitting up, however in the next shot from overhead, the boy is now lying down on the rock.
Continuity mistake: The students' 3rd game is Kagome with 4 Kokeshi dolls. The pink doll starts to the right of the first student to play, the dolls move in a circle around him, she passes in front of him and by the end of the shot she is near his left. The Kokeshi dolls all move backwards between shots and the pink doll passes in front of the student again. After this, she takes 7 seconds to move about 100° despite nobody slowing down and she took that amount of time to move about 170°.
Continuity mistake: In the middle of a sentence, new text appears on the chalkboard behind the speaker, as well as new colouration on a globe behind him. (00:32:42)
Suggested correction: I've changed my mind about this entry. I realise now that these seeming errors are actually intentional, albeit hamfisted, visual manifestations of the changing timeline. I personally think this is a silly way to portray the changing of history due to time travel and the butterfly effect, but who am I to argue with how a fictional construct that seems to defy logic would play out. I'd remove my original entry but I don't see any mechanism to do that.