A lot of the Nazi rally imagery in Leni Riefenstahl’s Triumph of the Will shows up in other, newer documentaries, but I doubt too many folks have watched the original.
Tom Cruise’s dance in Risky Business. Not only has everyone seen it, it’s definitely on the list of most parodied film scenes.
There are some bad movies with terrible action scenes that a fair number of people have seen now, thanks to the youtube. I’m thinking of this one (which has nearly 1.4 million views) and the pommel horse sequence from Gymkata.
Taking this to a crass level, lots of people have probably seen Phoebe Cates taking off her bikini top without having seen the rest of Fast Times at Ridgemount High.
The same is probably even more true about Chloe Sevigny performing fellatio in The Brown Bunny.
Most people have seen the image of the [url=http://3.bp.blogspot.com/_jalm22zr-80/TPHZ2WibFcI/AAAAAAAABJY/uUzl0WhrxzM/s1600/atriptothemoon.jpg]bullet in the moon’s eye from A Trip to the Moon, and the sequence leading up to that has been shown in every documentary about SF films. But the entire film is rarely seen.
It’s possible that more people have seen Clark Gable saying, “Frankly my dear, I don’t give a damn” than have seen Gone With the Wind.
The “razor in the eye” scene of Un Chien Andalou is the only part of the film that most people have seen.
Well, not quite everyone - when I clicked on that link just now was the first time I’d ever seen it.
Meg Ryan’s diner-gasm in “When Harry Met Sally”.
The scene where a house frame falls over Buster Keaton, but a strategically placed window frame keeps him from getting hurt. It’s from** Steamboat Bill Jr.**
Except for that BoaN was shown on TCM last week.
“Stella!”
I would say it’s certain, as the set of people who have seen that scene includes the subset of people who have seen the whole movie. 
Deliverance (1972). I think more people associate to the type of people in the movie (“backwoods/hillbillies”) and the rape sceen than have seen the movie.
Probably not exactly in line with the OP but…
I bet a lot of people watching Zero Hourwould have this awful feeling of déjà vu.
Forgive me if this is wrong, but I’m going to assume you aren’t aware that DeNiro was copying Alan Ladd from Shane in that scene.
Taxi Driver is very well-known. Shane, apparently, is less-so.
Before I watched The Shining, I had already seen about 90% of its scenes parodied in some fashion.
Yeah, I’d say it has more to do with the fact that a) it’s ancient, b) it’s B&W, and c) it’s silent (kinda all three go together in this case). Anyway, I think most programming types would consider that demographic poison.
Doing my best to correct this, TotW is assigned viewing in my AP Euro class.
At work so can’t click on link, but your title reminded me of Zero Wing, which was a video game and not a movie, but which fits the thread otherwise for its All Your Base intro.
Did you like the movie Airplane! ? (Have you ever been in a Turkish prison?)
Click the link when you can.