I only noticed this last night when the game finally came out, I started playing single player when a couple of my mates wanted me to play with them, so i joined them and they were up to act 2 and were further in the story line, The side quests do not count towards me as i am not that far yet, but the best thing is when we started doing the main story line it was counting towards my progress which i did not know yet, after playing for a few hours i decided to play by myself thinking i would still be in the same area I was before and knowing whats going on BUT!!! With the main story missions counting towards my progress i continued from where my friends last main mission was and not where i last was, so now i've skipped alot of stuff in the story line.

So pretty much to do this glitch you need:

Internetz
Friends or Randoms who are further than you in the story line
And time

as far as i know, i am teh founder!