The commentary says that no one ever gets this joke. We first see Rex Kramer dressing in front of a mirror at home. We cut away to the dog attacking the trainee gag. Back to Kramer and it is no longer a mirror. Genius.
"'Extremely High Voltage.' Well, I don't need safety gloves, because I'm Homer Sim--" - Frank Grimes
How can people not 'get it'? I mean, there's not a huge obvious punchline, but the joke is really just that they pulled a switcheroo on us when they cut to the dog humping the guy with the car.
Of course the reason is that a movie wouldn't work like this today is because spoof movies insist on the point if every gag being explained or obvious to the viewers. It's why today's spoof movies almost have a subtitle on each joke's screen telling you exactly what movie it's from...
My cousin had to show me that scene three times before I caught on. And I'm usually very good at picking up gags like that.
I am - SUPERFLUOUS!!!
I always loved the joke. My guess is that it's a real IQ tester of a joke. Those of us who are perceptive don't often realize how much less average people analyze what they're seeing. A lot of people spend more time listening to the dialogue in a movie than they do the visuals of the scene, especially if the visuals present themselves as mundane details. So when something like this happens that depends on remembering what they saw a minute ago, as opposed to something they heard in the dialogue, they are lost. They weren't paying attention to the visual set-up of the scene because it wasn't portrayed as the focus of the scene.
"Pretentious" is a three-syllable word for any thought too big for little minds.
This gag foreshadowed a number of genius gags of this type in their next movie Top Secret! (1984).
It's pure cinema, I love it.