Continuity mistake: Enter The Cheatrix: After Bart and Lisa break into Frink's basement, they discover a Rift Portal. The first shot shows the "DO NOT USE" sign a foot above Bart and Lisa's height. But as Bart walks into the portal, the sign is suddenly located at the same height as Bart.
Chosen answer: As seen in the film, the portals are in flux. They are constantly activating and deactivating, potentially pulling items through from one dimension to another.
MasterOfAll