Changing the default skin used to give Error 500 on all pages. This was fixed by AO3-4738. However, changing the default skin now causes all users to see the Archive unstyled, although with the color set of their chosen skin.
How to replicate the issue:
1. As an admin, go to Admin > Skins.
2. Scrooollll down to the bottom of the page and hit the "x" next to "Default" (the usual default skin).
3. Enter the name of another approved skin into the box.
4. Hit "Update".
5. Note that you and everyone else has lost styling (it can apparently be recovered by setting the default skin back and clearing memcached).
Make sure that the new skin is included in subsequent page loads, both for logged out users, and for users with a custom skin adding onto the default.