At the beginning of the movie, a child is seen writing a letter to Santa and on the envelope it says "Timmy House" as the address in the top left corner. When the letter is seen at the post office it instead says "Jimmy 215 Elm st.". And the envelope is a slightly different shape.
While Matt and Kristi are talking in the corridor at the Office, his shirt's top button is done up in one shot and open in the next, then back again, altering the position of his clip on tie.
When Matt and Kristi go for hot chocolate and Matt reaches up for a can of instant cocoa, the can is mainly white with writing. In a later shot, the can has changed and has a green band at the top.
The sprinkles in the "sprinkle fight" scene are very obviously animated and edited in in post.
When Matt and Kristi meet in the park, all trees are green and in full bloom although the movie plays around Christmas.