In the episode "Mr. Griffin goes to Washington" when they are at the baseball game look closely at where everyone is sitting. At first there is an empty chair then Peter then Lois then Stewie.

And then Brian then Meg then Chris then an empty chair in front of them. Then after the Tyson cut scene the chair next to Peter is gone and the person next to Stewie is different. Then after Peter falls down the stairs and runs into his boss he comes back up and sits next to Brian. There was no seat next to Brian.(the empty seat next to Chris might be gone also)