The food stain Doug leaves on Joyce's jacket changes size and color between shots.
When Bernard (Hal Linden) plays the piano during Arthur's (Jerry Stiller) dinner party, you can clearly see that his hands don't touch the keys.
When Doug catches the foul ball at Shea Stadium, there's a close-up of the baseball in his hand with the label facing out; it is not an official Major League baseball.
When Deacon, Doug and Carrie are leaving the house to go to the Mets game it is already dark outside. Even for games in the spring or the fall it wouldn't be dark out that early in New York.
Joyce is on the phone outside the Mets suite. She walks into the suite and the door closes. When inside the door is open.