The set for the Heffernans' living room includes windows on both sides of the front door (the front door is centered between windows), but the view from outside of the front of their house shows the front door next to the right corner of the house with no window in between on that side (all of the windows are to the left of the door).
When the outside view of the Heffernans' house is shown, there is no porch. Yet there are multiple episodes where they are on their front porch.
The exterior shots of the Heffernans' house from the street shows a screen door on the front door, but when people go in and out of the house on the set, there isn't a screen door. (Note: screens are never shown in the interiors of sit-com houses; outside the doors or windows.
In one episode, Carrie (Leah Remini) and Doug (Kevin James) talk about "their" song in the ninth grade. In another episode they indicate Doug met Carrie while he was a bouncer at a local bar. Further, the two actors are 5 year apart in age so they could not have been in the 9th grade at the same time.
In the episode where Carrie boycotts Doug's favorite restaurant, Arthur receives a VHS box set on the Civil War (blue and red box). The boxed set is changed to "The Great War", World War I...different color (gold and white) and smaller package, too.
In episode 20 of season 7, 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.
In one episode Doug and Carrie's wedding is shown. Here you
obviously see Carrie's wedding dress. In a later episode where Carrie has become concerned with her weight, she is shown trying to fit back into her wedding dress. They are two completely different dresses.
In several episodes when the camera angle is in the kitchen facing the door to the living room and the window, there are times where the shot is wide and you can see there is no wall and can see right to the front door of the house on the set.
In a flashback episode where Doug is first working at IPS, a locker with Danny Heffernan's name is seen. Danny didn't start working at IPS until long after the flashback took place.
In several episodes with scenes that take place outside, palm trees can be seen in the background due to the show being filmed in Los Angeles. There are no palm trees in New York City.
In some episodes, characters from the show are shown with paper cups from the restaurant In and Out Burger. There are no In and Out Burger restaurants in New York City.
In a few episodes there is a reference to the characters going to "Dairy Queen". There are no "Dairy Queens" in Queens.
In one episode, Doug indicates that his birthday is February 9, 1965. In another episode, Doug states that he is a Pisces. If his birthday was February 9, Doug would be an Aquarius, not a Pisces.