When Billy has two black eyes and is talking with his dad, the shape of his shiner on his right eye changes. At first his black eye is right around his eye, and then the bruise is from his temple to halfway down his cheekbone.
Billy puts a headband on one Christmas Day; it's missing in the sequence when his uncle runs into the garage, gets out, and checks for damage.
On the first Christmas, a little boy guesses there are 2459 jelly beans in the jar...but Joey wins with a guess of 2456 jelly beans. On the last Christmas, Billy adds 3 jelly beans to the jar. The little boy wins with a guess of 2456, the same that Joey won with on the first Christmas.
In the end of the very first day of the series there is a fight in the church when Joey hits Billy with his left hand. But later the bruise on Billy's face is seen under his left eye as if he has got a right hand blow.
When on the second day Billy and his father come out to the basketball ring in front of the garage and Billy tries to say that it already took place before, camera several times switches between father and son and each time the ball is either in father's or in son's hands as if they were passing it to each other.
When Uncle David skids his car prior to hitting the garage door, squealing tires can be heard. This would not happen on snow.
Sunrise on December 25 in Virginia is not until around 7:23 AM, so at 7:00 AM the sun would not be shining into Billy's room like it does in the movie when he wakes up on Christmas morning.
Full green trees are in full view in the background when Billy is at the basketball court playing ball with Joey and the others, and also when he is practicing his jump shot in the front yard.
SPOILER:..On the last Christmas...Santa shows up outside Billy's house on Christmas night. Santa should've already delivered those presents on Christmas Eve and be back at the North Pole resting.