Continuity mistake: In the scene where Bernard the elf appears at the Miller's house near the end of the movie, there is a shot of him walking out of the kitchen with Neal's snack that the latter had been preparing earlier (a stuffed pita shell) and he's talking to Neal as though his mouth is full from having taken a bite. However, if you look at the pita, it is still whole - not a single bite taken.
Continuity mistake: When the sleigh makes the vertical take off, Santa asks Charlie if he's ready to go. We see both Bernard and Quentin look at each other and smiling as well as all the other elves behind them. In the next shot, not only are none of the elves smiling, but they also are all wearing welding goggles. (01:14:00 - 01:14:40)
Continuity mistake: Right after the part where Santa fell off the roof and Scott is yelling at Charlie to stop climbing up the ladder, you will notice their roof is slanted and has a significant amount of snow on it. However, from the top of the ladder Charlie exits off with complete ease (given the circumstances of the roof and ladder position, difficult for a skilled adult let alone 6-year-old Charlie) and walks onto the roof, and the roof portion is now obviously a flat surface.
Continuity mistake: When Scott and Charlie are at Denny's, Scott picks up the eggnog placard from the table to ask the waitress about it. He then puts it back down in a different spot. However, in the shots of Charlie by himself (such as when he asks for chocolate milk), the placard is visible in the foreground, in the same place it was before Scott picked it up. (00:09:55)
Continuity mistake: After being told that he cannot see Charlie till the next year Tim Allen steps into Charlie's house. Before he enters it's snowing, when you see him inside with Charlie in his arms you can see outside the dining room window that there is no snow, and when Charlie's mum goes outside it's snowing again.
Continuity mistake: When Scott Calvin hears something on the roof and he goes outside, he sees Santa on the roof. He calls to Santa, "hey you", and Santa turns around and slips. As he's sliding down the roof, and falls you can see where Santa slid on the roof, because there is a line of no snow, in the next scene, like five seconds later, there is snow where Santa slid.
Continuity mistake: On Christmas Eve, Scott runs out of the house to check the roof after he and Charlie hear a ruckus. Right as Scott sees Santa and yells "Hey You!", take a good look at the roof and the snow patterns just to the left of where Santa is and where he slips. Water and "wet" snow melt patterns also indicate an icy and probably a slippery surface. A few minutes later in the movie when Charlie and Scott ascend the magic ladder to find Santa's reindeer, the roof setting and snow patterns that were to the left of where Santa was originally standing are now completely different (and a flat, non slippery surface, no wet patterns), indicating a safer stage set.