This starting appearing in 4.0 beta series. I'm using the official builds, and sometimes, some items (images, css) get cached, and when accessed later, are not valid anymore, so they're not displayed / used. This results in a page without CSS, or with missing images. I attached a screenshot of the issue, as it's kind of hard to explain. Hitting Ctrl-Shift-R fixes the page, which leads me to think it's a cache issue. I was unable to find a similar bug in my searches.

Please see e.g. bug #588804 and bug #596443 for similar issues. If symptoms sound familiar feel free to duplicate this bug.

Thanks Bjarne, I was hesitating to file, but I decided do to it anyway, just to be on the safe side. Sorry for the dupe!

No need to be sorry! With this sort of thing, it's better to make sure it's filed.