As long as we're talking about BTTF paradox, what about Doc Brown's answer to Clara Clayton when he said he had to leave her and told her he couldn't take her with him. She didn't yet know he was going back to the future, and he felt he couldn't take her out of her time.
But wasn't that the absolute wrong decision? In the original history, Clara was meant to die in a horse-driven wagon fall off a cliff (that came to be known as Clayton's Ravine, remember?) When he saved her, she no longer belonged in 1885. If he went back to the future and left her there, she would change Doc's history. As a young woman, she would probably marry a man.....who, in the original history, would have married a different woman, thereby creating different children than history intended be born. Those "new" children would create alternate unions with other spouses, and still more children who were never meant to be. In a pioneer town like Hill Valley, the entire dynamic would be changed by the time Marty McFly was born.
To add to this paradox, Doc Brown and Clara created two children, Jules and Verne, who were never meant to exist at all. These two kids would eventually mate with townspeople and royally screw up the future.
How about this example?...... Say Sheamus and Maggie McFly have a daughter back in 1885 and instead of whomever history intended she marry, she instead marries Jules Brown (son of Doc and Clara)?
What then, huh? After all, if Sheamus and Maggie's daughter doesn't marry the guy history intended she marry, none of the McFly men who existed in 1985 (George, Dave & Marty) would even come into being! Other McFly children would be born, but not the ones we knew. So, when Marty goes back to the future, it's a future in which he never existed and can't exist now.
It's ironic that Doc Brown, the man who was so intent on not messing up the past, was solely responsible for doing just that.
As Marty McFly would say: "Holy shit!"