Generally isn't a matter of being "full"... it is just sometimes a page gets "stuck" and won't update like it is supposed to if the one on the web server is newer than the one in the cache.

There is a bug in IE though that when the cache gets too large, and you try to save images from a page, it will only save them in one format. No matter what the image type (.gif or .jpeg) it will only let you save it as .bmp. In that case clearing the "overstuffed" cache solves the problem.

Stupid IE.