I finally found a way to reproduce this problem. Normally there is a "Full Version" button at the bottom, but if you click it and switch and then click the "Use Mobile Version" link at the bottom to switch back, the "Full Version" button disappears. It only comes back after clearing cookies or waiting for the cookie to expire.
The actual use case is that I like the mobile version but it doesn't show people's location which I like to know when browsing the Classified area, so I switch it to full version. But then if I switch back to mobile for viewing the rest of the forum, I can't switch back without clearing my cookies.