Grid parts overlapping

Hello.
I created a responsive grid of 3 parts (`pure-u-1-3`). When I resize the browser's window, the content of two of these parts overlap resulting in a very unfair layout.

I reproduced the issue here: http://jsfiddle.net/Zj5Aq/
You can play with the demo here: http://jsfiddle.net/Zj5Aq/embedded/result/
