Until this episode it was always said that when Derek found Mark and Addison together in bed he just turned around and left without doing anything or saying anything to them. But in this it when Derek and Addison reminiscing about the past it shows Derek throwing out all Addison's clothes and her out of their house after he caught Mark and her together.
When Addison is talking to Richard about how to determine the real mother she said they could do a blood typing, that because the baby is 'double O' that if the girl is A or B she can't be the mother. This is untrue. The only way that it is not possible for her to be the mother is if she has AB blood type, as A blood can be comprised of AA or AO, same with B. If the A type blood is AO, then the mother could give that O gene to her baby.