"You may use coincidence to get your main character into trouble...but not to get him/her out of it." I wish I could remember the exact interview linked through GITS where I first saw this. I think it may have been Chris McQuarrie. Again, sorry I can't recall the source...

I find it odd (and I'm trying not to find it depressing) that in 93 years, we haven't collectively moved past making the same mistakes Anita Loos saw in 1920. What does that say about knowledge transfer and organizational culture in the screenwriting community? Also, while we all see ourselves as the heroes of our own stories, I think many people see life as something that "happens" to them rather than seeing themselves as agents or active players in their own lives--and that can be reflected in how they tell stories when they sit down to write. Over the course of the past year, when I was writing script notes, I had the occasion on a remarkable number of scripts to point out that the hero is the person who makes the ending happen--so if that person is not your protagonist, either you have the wrong protagonist or the wrong ending. (A related problem that overlaps with the problem of coincidences.) John, I've seen that quote, too, attributed to multiple sources. Good thing, because a lot of comedy wants access to coincidence to cause trouble--especially farce and door-slammers. And that's still about setting off your story or escalating the trouble--ultimately, the protagonist still needs to take action and deliberately make the ending happen.

It means that in 93 years, it hasn't gotten any easier to tell a good story. That was true 1000 years ago, and will be true 1000 years from now.

It was one of Pixar's rules, and it's a great one. Better than that one coincidence rule. http://io9.com/5916970/the-22-rules-of-storytelling-according-to-pixar