Revealing mistake: About seven seconds after Mercedes screams, "No!" at the climax of the movie, there is a shot of all the characters involved in that scene. If you look out the door you can see a white car drive by. (01:59:40)
Revealing mistake: Very near the end of the movie, when Edmond and Fernand are about to start fighting, a power line is visible in the background as the camera pans over from Edmond to Fernand's back.
Answer: I believe it's just because there is a dirty smelly man that she doesn't recognise visiting her ailing grandfather, she just wants to keep an eye on things.