So I went and updated a line in layout.css twice. Then, I undid those changes. I'll call the different versions "CSS original, CSS 1, CSS 2".
Now I'm experiencing a problem where my browser is using the wrong css. I cleared my browser's cache, but the problem still persists. I thought it might be caused by server-side caching, but for some reason different browsers are using different versions of the same CSS: Firefox is using CSS 1 and Chrome is using CSS 2. I have no idea what could be causing this problem, and this happens on different computers too.
EDIT: Found the problem, it was caused by a plugin called "Supercacher". I purged the cache and it works now.