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.
Dr. Addison Montgomery states that their only option is blood typing: "The baby's blood type is double O, if the girls are 'A' OR 'B' it means the baby is not theirs". This is completely false. Firstly, blood type "double O" does not exist, it's just type "O". Secondly, if the baby is blood type O, it means the mother can be either A, B, or O, just not type AB (NOT "A" OR "B"). We cannot narrow down who the mother is unless we also know the father's blood type.
The plague in modern days is highly treatable. The reason it killed so many in the dark ages is because no one knew what they were dealing with and how to treat it. And in other countries such as Africa they lack the needed medicines. But Modern medicine in the US can take care of it. Also the plague takes a few days to kill you, if it even does, all those exposed had nothing to fear. In short, the plague was blown out of proportion.
When Derek and George are quarantined in the locker room, they start ransacking the lockers looking for food. They open the lockers simply by pulling up on the handles. But it's clear that all the lockers have built-in locks. Also, the edge of the doors should have holes exposing the latching mechanisms. Instead they're all solid, with no means to engage latching hooks. This all clearly shows that these are prop lockers, not real ones.
In a previous episode, when Richard and Ellis were reminiscing, one of them said it was raining the day at the carousel, but in the flashback in this episode, it was not raining.