Continuity mistake: Throughout most of the episode, the repeated conversations seen in each character's stories are slightly different to each other: the way certain lines are delivered, the position of the actors, etc. These scenes were most likely re-filmed.
Revealing mistake: In Cameron's Story, in the scene with her and Derek, Derek's phone rings. When he looks at the screen it shows a telephone number and the words "EDIT" and "BACK" in the bottom corners. The phone is not in fact ringing at all. Derek is merely looking at the details of a contact saved on the phone.