One of the most common mistakes I see in web development is not adequately testing sites for a variety of viewport sizes. It’s more than just incorrect rendering on mobile devices, or looking bad on large screens. Issues with small browser windows are often overlooked in the course of building a site.
I notice this almost daily in my development environment, where I work with two 960px windows side-by-side. In so many sites I visit, I know if I see a scrollbar on the bottom of the page, I’m likely to see a min-width error when I scroll to the right.