Especially science-fiction films where extraordinary events changed the status quo. Sometimes I think about “ok, so what would that have meant for the future?”. For example, Them, or any of the other “giant mutation caused by atomic testing” movies. Think maybe that was the reason a test ban was signed in the 60s? 
War of the Worlds is another good one, except that the TV series (which you may or may not accept as canon) postulated that somehow a mass amnesia caused most people to forget or dismiss that it had ever happened. :dubious:
My favorite oddly enough is Five Million Years to Earth. After the events of that movie, humanity knows that Mars was once inhabited by intelligent life and that their artifacts can still be active after all this time. I keep sorta picturing what the first manned expedition to Mars would be like when they KNOW they could run into more artifacts like the buried ship.