The Woodstock documentary, when it was originally released. I was too young at the time to actually be interested (I was like 8 or 9 years old). Saw it again when I was in college after a night of drinking, and don't remember much of it from then either!
I keep intending to watch it again sometime, but haven't gotten around to it yet.
Aside from that, it was probably Heavy Metal (also while I was in college).
Trivia tangent: The Blue Oyster Cult album Fire of Unknown Origin
has several songs on it which were written for the Heavy Metal soundtrack (one is even a musical plot synopsis of part of the movie), but which ended up not being used. The one BOC song that did
end up on the soundtrack was Veteran of the Psychic Wars. IMO this is one of the best albums BOC ever released, and might even make my "desert island discs" list (depending on the length of said list)...