Lol, I fell asleep in the middle of Citizen Kane

; ditto the first time I tried to watch The Maltese Falcon (which I have since seen).
Of the films mentioned on this thread, I have seen Casablanca (I was 17 at the time and thought I should have seen it much much earlier for some reason) and Gone with the Wind - but I haven't really seen many of the classic films. In fact, I haven't ever really tried to watch them. The only reason I've seen Ben Hur and (most of) Lawrence of Arabia was that they were conveniently broadcast on telly. Haven't seen much Hitchcock (apart from Psycho, the film where the keys become central and the one in which there's some confusion about a woman's identity or something - what a great impression they've had on me

). Think I've seen One Flew over the Cuckoo's Nest, but
haven't seen Godfathers, In the Waterfront, 1960s and 1970s musicals (such as West Side Story, Grease, Saturday Night Fever)... OK, let's face it, I've probably seen practically none of the classics made between 1950 and 1990 (assuming films need some time to become classics)
French cinema - my teacher made us watch Les quatre cents coups at school; for some reason I saw Le grand illusion once (around the same time I saw Casablanca) - but that's about it, I couldn't even name any other films
I guess there's never time enough to see them all. And I guess we'd need a definition of a classic too...
Often you have seen snippets and read/heard about the details of the films, so when you get down to watching them, you actually know quite a lot (or think you do

)