The window Arthur found in the wall over his bed in "Bed Spread" is not there anymore.
When Doug is at Carrie's law firm picnic and is playing softball, he hits a long fly ball and tries to score by running over the catcher, who is waiting at home plate with the ball to tag him out. Doug knocks the catcher down and the ball is seen lying quite far away from the catcher, as shown from the view above. When a close up of the catcher is shown, the ball is now lying right next to his head.
The park that the softball games are being played in is located in New York City and yet there are mountains in the background.
When Arthur is talking to Deacon in the basement he claims that the only other black people he knows are on TV, which is not true. By this time, Arthur was very familiar with George Barksdale, who was also black, from the senior center.
Deacon says to Carrie that he's never been down to the basement, which isn't true.