Well, there are SOME events when it is really the last - and I mean really the last - thing you need, and that's when you have it -.- When you now it in advance, go to your GP and ask for a special pill, it is not the regular anticonceptiva pill, but something slightly different, don't know.
When my aunt died, though, I didn't get it for over 3 months. Scared the hell out of me, I thought..well... =) So, stress can totally mess up your whole cycle, and that kinda sucks sometimes.