Continuity mistake: When Harry is meeting the boggart it before it turns to a side view look at the girl to the left of Hermione, she is laughing, it changes to the side view and she is just standing there. It happens with Lupin too - in the side view he is playing with his wand then it switches to him and he isn't.
Continuity mistake: When Harry, Ron and Hermione are in the shrieking shack Lupin comes in and says "Finally the flesh reflects the madness," look at the left side door behind him, it is facing outwards then it turns inwards in the next shot of him.
Revealing mistake: When Hagrid stands up and nearly knocks over the table, McGonagall's goblet manages to stay rooted to the spot.
Chosen answer: Fred and George used the map to aid them in their mischievous activities at Hogwarts, and probably had little interest in what their little brother was doing or who he might be with at any given time.
raywest ★