When Tommy installs the plant Mr. Merrivale gifted them, he drops the empty pot inside of the single stack of pots which are several inches away from the planter wall. When Dennis and Tommy return and approach Mr. Wilson and the group to confess what happened, there are now two stacks of pots which are now flush against the planter wall.
When Mr. Wilson steps upon the stone patio after planting his philodendron, the edge of it is curled clearly showing it is thin linoleum and not stone.
Visible wire attached to leaping frog.