The back button should be a stop button if a page is loading.
If I'm on page N, and click a link for page N+1 that I decide I don't want to visit - maybe it takes too long, maybe I don't like the URL, maybe I spied a better link on page N, maybe I tapped it accidentally - I want to hit 'back' and end up on N, not N-1, and not have to hunt around on a toolbar to find the X that does it.
Someone please make this for every browser in existence. I would pay to use this on every computer I own.