In the christmas episode, when frank comes in as santa, then carol comes dressed up as him too, you'll see mark come in the front door as santa as well. Carol saya "well who do we have here?" and then the camera cuts to frank sitting down, and he says "Well who DO we have here?" but if you look to the right of his face, you can see a shadow (like a character waiting to enter) on the swinging door that leads to their kitchen. Then, after lilly leaves, you see Al come in through that door saying "HOHOHO! Aww forget it!"

Not a huge error, but a fun little slip-up I noticed.