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Usually what happens is you click mark forums read one time, either accidentally, or just messing around, it becomes a saved setting. If deleting the cookies and the temp files don't fix the issue, then check back.
Either way, this is going to the help center. Not the tech forum
its been doing that a little to me as well. i just look at the date and time of the new posts to see where i left off.
i've also noticed that if you go back to the 2nd gen page a few times without clicking on a thread then i will eventually not be in bold any more.