It never ceases to surprise me when I find out that a movie that's so well-regarded by modern generations that it's attained the status of classic was actually a flop or sometimes even hated at the time they came out.
Just a few examples include:
It's a Wonderful Life
The Wizard of Oz
The Thing 1982 Remake
Blade Runner
Citizen Kane
12 Angry men
Vertigo
Willy Wonka and the Chocolate Factory
Yet now these movies get nice, fancy home video release after nice, fancy home video release, constant reairings on television, the net, and rereleases in theaters.
What makes people change their minds on these kinds of things?