Revealing mistake: When Uncle Louis drops the match in front of where Eddie empties the "chemical toilet" if you look VERY close you can see the rope that launches him into the air. (01:32:05)
Continuity mistake: When Uncle Lewis lights the cigar that sets the Christmas tree on fire, watch closely. It shows side shots of Clark eating his dinner, and you can see the tree in the living room. The camera cuts back to Lewis lighting his cigar, and then back to Clark at the exact same angle as before. The tree is gone. (01:11:10)
Continuity mistake: After Clark receives what he thinks is his bonus check, he says if there is enough money left over he will fly all of the relatives out to see the new pool. Everyone reacts with excitement, particularly Ellen, who is standing right next to Clark. She lets out a cry of excitement and raises her arms. The next shot is taken from the opposite point of view (from over Clark's shoulder). In this next shot, you can see everyone else still excited with the sudden exception of Ellen. She went from shrieking in excitement to having her mouth closed and her arms back down at her sides.
Continuity mistake: When Clark's uncle drops the match that sets the sewer on fire, his toupee falls off and at the end of the scene you can see him trying to put it back on, although it's burnt pretty badly. In the final scene where everyone is dancing around, his toupee is back on and in perfect condition. (01:32:35)
Continuity mistake: When the squirrel jumps out of the tree and the whole family is hiding around the corner in the front entry, the camera alternates shots frequently from the front of the family to the back of the family. Each time the angle changes, Catherine's stance changes from her arm being around Russ to her arm at her side.