If you have more than one tag open in IE 7 the browser sometimes degrades the graphics on the one you are not looking at! For example, if the website you aren't looking at has a gradient which is normally smooth and well done IE degrades the gradient to be poor quality and has solid colours for 10px at a time!
So is this some kind of clever way of saving temporary files or is it some kind of bad accident. In my opinion it really makes some sites look bad and for some reason it doesn't do it on a regular basis!