12 More Movies You Didn't Realise Stupidly Broke Their Own Rules

5. Biff's Time Travelling - Back To The Future II

The Rule:

In Back To The Future, if you change the past, your actions have huge impacts on the future. Something as insignificant as destroying a tree has an echo in the future, and while you're aware of the changes, nobody else is. Simple time travelling logic.

The Breaker:

Biff Tannen is the issue here, as he usually is.

In 2015, Marty McFly buys himself a Sports Almanac with the intention of making his past self rich. He's admonished by Doc Brown, but unfortunately for them both, Biff witnesses the whole thing, picks up the Almanac and somehow manages to pilot the Delorean back to 1955 to give it to himself, then returns to the exact same time so nobody notices.

By the laws firmly established in this film universe, this is impossible. The second Biff went back, he irreversibly changed the timeline, and the future that he goes back in time from no longer exists. With the new 1985 timeline established - in which Biff becomes Donald Trump - the old 2015 is wiped out, so he wouldn't be able to get back to where he was.

He should have arrived in a completely different dystopian 2015 in which Biff had grown to old age as a billionaire and was probably president or something silly.