Now here's a paradox that, in theory, should derail the entire franchise in one fell swoop. So by messing with the past so much in the first film Marty returns to a 1985 that's almost unrecognisable to the one he left. Yeah there's the hi-larious Lone Pine Mall gag and the thing with the clocktower, but his family life has improved to an insurmountable degree. He's now not living with an alcoholic mother and a deadbeat dad in a crappy house on the wrong side of the tracks, but he and his siblings have apparently grown up in the lap of luxury, with George as a bestelling author and Biff as his emasculated servant. It's pretty sweet. It also breaks Marty as a character. Marty presumably didn't have a great time growing up. All of those experiences lead into his character, though: he didn't do well at school, so he hung out with Doc Brown; his family couldn't afford a car, so he skateboarded everywhere; stuff like that. Make his entire life a cake walk like it was in the fixed timeline, and none of those time travel shenanigans should ever have happened. There's a whole bunch of ripple effects, but take one for example: he probably grew up in a family that could afford to buy him a car for his sixteenth birthday, so he never got used to skateboarding, so the skateboarding bit in 1955 couldn't have happened, so he should've been pounded by Biff and his gang. And so on.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/