If the plot of Back To The Future happened in real life, the results would be catastrophic. That's what Will Fink thinks, anyway. If Marty met George, that would prevent George from having Marty, but then if Marty was never born, he couldn't prevent George from having Marty, which seems like a major paradox.
Super confusing, right? This is probably why we don't have time travel, and never will.