In the establishing shot for this story, there is only one tree in the Flynn-Fletchers' back yard. Later in the story, Phineas and Ferb are having a tree house fight with Candace and Stacy, using two separate trees. Finally, after their mobile tree houses leave then return back to the back yard, there is one tree in the yard just as they return, bringing the total up to three trees.
During the van ride to the kids' grandparents' cabin, the number of seats on either side of the van keeps changing from one, two, or three seats between takes.