I wanted to add the same idea, but then found it already here. I don't know if it was in 2013, but currently there is also auto margin left.
I'm cleaning these auto margins every time, but then I can move some div and - ta-ta - it appears again without me noticing!
This is really annoying that you have to watch for those auto-margins appearing every time.

This might be unnoticed in a common web app, I suppose most people just don't notice this behavior if they use standard themes and mostly desktop screens, but when you are developing mobile application, optimized for fat fingers, with your own theme, those sudden 10 or 20 pixels can actually spoil things.