When Mr. Douglas squeezes the spruce juice bulb at Druckers, it's shown spraying out of the nozzle and there is a red light at the top of the tree. The tree at Druckers isn't decorated. The same top shot of the spruce juice spraying out is shown at the Ziffels. Their tree has the red light on it. Showing that Druckers store shot was shot on the Ziffels set.
Near the end when everyone is at the Douglas' s to see the live tree, Lisa is wearing a dress that has both shoulders exposed with a red shawl to cover them. Each new camera shot of her at some point has either the right, left, both, or none of her shoulders covered.