Other mistake: Keira Knightley looks down at her phone, there is a beautiful female hand holding the phone with long slightly curved nail beds. This is not Keira's hand which is seen many other times times throughout the film. She has shorter nailbed and flat nails. (00:22:37 - 01:13:56)
Continuity mistake: After Dan meets Gretta he walks her to the subway. A girl with a cell phone is coming up, but in the next shot she disappears.