My wife discovered tonight that Mozilla (including Firefox) can't handle CMYK-format JPEGs. Apparently there aren't a whole lot of these (they are usually RGB), but apparently it is a valid part of the JPEG specification. I understand people who do prepress-style work deal with these.
I was under the impression that Mozilla used a lot of image libraries to handle the different graphics formats (like libpng, etc.). I would have thought that whatever library Mozilla used for JPEGs would specialize in whatever JPEGs need and handle it just fine. Instead the user gets a broken image icon, or if trying to view the image directly, a statement saying that the image cannot be viewed because it contains errors.
This bug was reported four years ago.