When designing a website, a Web Designer needs to consider that there are over 100 web browsers; and web pages can look very different between the browsers.
To make things more interesting, some browsers, *ahem* Internet Explorer, have compatibility issues even between their own versions.
As an example, a designer could spend hours ensuring a design is compatible with Firefox, Opera, and Internet Explorer 7.0, then do a quick check in Internet Explorer 6.0, revealing a completely new issue.
Though most people use Firefox, Opera, and Internet Explorer, a designer should remember the less popular browsers when designing a site.
A great tool [free] and enormous time-saver for Web Designers to check Browser compatibility in multiple browsers is BrowserShots.