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I've always classed Cult as being a box office failure, or at least only making a slight profit, but actually turned out to be a pretty good movie which built up a solid following through TV showings and video/DVD sales.
Dredd for instance, and The Big Lebowski and movies like The Toxic Avenger, Dark Star and even stuff like Slither, An American Werewolf In London and The Rocky Horror Picture Show.
But, yeah I guess BTTF can be classed under Cult if you go by the fanbase. It's the sort of movie with a following that is incredibly loyal.
Star Wars as well then I guess too.