Say you have a webpage, with a background image and a select box. In Internet explorer 5.5, you will run into an issue where the background image won't fully load, and the contents of the previous page will be your background instead. Moving the scroll bar will sometimes fix the problem, or sometimes make it worse.
That is, if you are using IE 5.5 in stand-alone mode, along with IE 6.0.
Which goes to show, the only sane way to browser test is to install VMWare or have a separate machine for every IE version you need to test. Yes thats right, a separate computer for every (Microsoft) browser that you need to test.
Insert heaping of scorn upon Microsoft here. Man. Just make your browsers stand-alone-able already.