This happens to me all the time. For some reason it won't purge by itself. When it happens, I also cannot right click and save as to jpeg images, instead they try and save as bitmap.
It's also happened to me a number of times with IE6. I always switched to another browser, figuring IE needed a break. When I switched back to IE all would be well again . I'll have to remember to clear the temp files next time. I like having the source code open in notepad.