Other mistake: Watch when Carol is singing "Oh Come All Ye Faithful," as she walks down the line of on-lookers, an extra in the back moves along with her and keeps looking at the camera.
Other mistake: Mike asked Carol where his checkbook was because he needed it to buy her a Christmas present. Carol pulled open the kitchen drawer she was conveniently standing next to, got the checkbook out, and handed it to Mike. A little later when Carol is telling Peter about her secret plan, she points out that she will be using their (Carol and Mike's) "special savings account." There would be a separate book for making deposits/withdrawals from the savings account, so it doesn't make sense for Mike to be using regular checking account to buy the present. (00:03:06 - 00:05:34)