Does Back to the Future Part II ruin the ending of the first movie?
In the first movie when George punches Biff to save Loraine in the car that was the last we saw of him when he was knocked out. So back in the present when Biff is working for George and submissive to him you assume it's because of the punch that knocked him out.
However in the second movie we then see Biff knocked out a second time by Marty, and then Biff just gets back up and drives his car and tries to scrape Marty off the road with the sports almanac in his pocket. So as we see now, the punch from George didn't change Biff at all because he's the same asshole when he woke up as he was before. So eventually Marty escapes with the hoverboard and Biff crashes into the horse manure truck again.
So why would Biff be so submissive to George in the present when the second movie now showed him getting knocked out didn't change anything about him? Are we just to assume George knocked him out again at some point off screen in the following days or something? George is still frail and weak and Biff is twice his size, so one lucky unexpected punch wouldn't work a second time.
Also this movie has other time paradoxes that don't quite work with the lore established either and Marty just leaves the 2015 hoverboard back in 1955 too.