The Final Countdown (1980) : The first movie to warn about the side effects of time travel to the past
I used to believe going back in time was the ultimate way to set things right and make everything better. But in this movie, an officer aboard a U.S. aircraft carrier which gets trapped in a mysterious light and travels back in time to right before the attack on Pearl Harbor completely broke that fantasy. He said that if you alter the past, you might not even exist in the present, and honestly, the more I think about it, the more he has a point.