The Back to the Future series is riddled with them. And I mean plot holes that break the very rules set out in the films.
Example: in BTTF we see Einstein sent forward in time by 1 minute as a test. The car and the dog disappear and then reappear a minute later and Doc Brown carefully explains that to Einstein it all happened instantly. Then in BTTF2 Marty and Jennifer go forward in time to 2015. This time, however, they meet or see future versions of themselves. Where did they come from? Since they removed themselves from the timeline by fast-forwarding to the future, where did these other thems come from? Einstein never met a future version of himself so why did they?
Another: in BTTF2 old Biff takes the Delorean back to 1955 and gives himself the Sports Almanac. Later in the film Doc Brown explains that this created a skewed timeline which led to the distopian alternate 1985. However, Biff still manages to return to the original 1985, despite having already skewed the timeline.
These are not just “suspend belief, it’s sci-if”, these are plot holes that counter the very explanations laid out within the films themselves.