When Doug takes Arthur to the buffet, when they are sitting at the table, you can see that the cup on the table constantly switches sides and changes from having a plate under it to no plate.
When Carrie pulls in the driveway with Arthur, she never puts the car in park, she just turns off the ignition.
As Arthur finishes humming the happy birthday song, he starts walking out of the living room. You can see the shadow of the boom mic streak across the upper right corner in the kitchen.