Deliberate mistake: When the grandfather clock chimes at 12:00, look at it when the clock ticks. It starts at 11:58 but then ticks to 12:00. The clock has somehow ticked 2 minutes instead of 1 minute.
Continuity mistake: When the boy re-enters the house, the grandfather clock reads 5:10. But when the boy is in his room, the clock reads 4:20. After waving to the Snowman, his clock now reads 5:05.