I appear to be able to see various threads that when I click on them, I get the error that the thread does not exist.
I'm guessing that these are threads that the OP deleted, but it may also be threads by new posters than need approval, or a combination.
e.g. In the Mathematics forum, theres Proof of Non-existence of Odd Perfect Numbers which is by yalc, who has no posts according to his profile.
While it's not really a big deal, the forum still considers such threads as "unread", and trying to view them doesn't get rid of the unread status. I'm pretty sure clicking on the ole "mark topics as read" button marks those threads as read, but that potentially marks things I will want to read later (i.e. things where approval is pending but it's legit) as read so I'm a little reluctant to do so.
I guess this is mainly as an FYI for the mods/admins in case it's something that can be easily fixed, but I'm guessing I'll just have to get used to it.