Character mistake: Meredith states that she cannot remember the last time her and Derek kissed. Derek says that the last time was when they were still dating, but it was actually at the end of season two episode three, after Addison arrived in Seattle and Derek and Meredith had stopped seeing each other.
Character mistake: Ms. Bitzer says she's booked a trip to Iceland to experience the sun never setting. While the sun does set quite late there, Iceland is located south of the polar circle and does not have midnight sun. And either way, season four has been established as taking place after summer, so there wouldn't be midnight sun anywhere.