Resize windows
![resize windows resize windows](http://www.quicktestingtips.com/tips/wp-content/uploads/2010/01/resize-windows-300x163.jpg)
Just look at the example of when I was writing this post. I love WordPress. I think it has one of the slickest and most intuitive user interfaces out there, and it doesn't even shrink well. As a general rule, when widgets overlap browsers get confused.
If you're testing a web app, whenever you get a screen of any sort of complexity, shrink it down or make it big and see what happens.