Grandpa and Herman fit the garage with two booby traps to foil anyone who tries to find out what they are doing- a trap door and a hook that lifts a person into the air. At the end of the show, when they fall victim to their own traps, Herman clearly has the hook attached to his back before we see it descend in the next shot.
When Herman is testing Eddie's present, the person driving is clearly a much smaller person. Herman can't even sit down in the seat when he first tries it out, but 'he' is seated normally during the test drive.
Before dinner, Grandpa reveals that Herman has cut his reading glasses in half. Grandpa then wears the "half glasses" to dinner; but since he's no longer reading, there's no reason for him to do so.