Originally Posted by jmwking
As far as I can tell, when you open a thread in the mobile skin, it should jump you to the first post you haven't yet read in that thread. That's a feature.
If you're logged in whenever you read threads, it'll remember your place, even from machine to machine and browser to browser.
If you're not logged in, things work very differently: it marks all threads as fully read shortly after you finish your session (via cookies on that particular browser), and the next time you open a thread, it assumes you've read everything you wanted to in the prior session.
One other thought occurs to me: if you have a wonky internet connection, and the posts page partially loads, stalls, and then auto-reloads, the system will think you already read it, mark the thread read, then reload to the end (or perhaps the next page of the thread if there is one). Neither a feature nor a bug, just an artifact of the system.