It's probably something like a topic that doesn't exsist anymore but that still has a link to it somewhere so that he gets that error message, or he's maybe got some other error message, that he's looking at. Something like that, I'd guess. The location he is at has not been defined, I reckon.
Nothing to worry about I think, it's not a secret place that's hidden from the rest of us. Well, I mean, there are places locked off for normal users, where only the staff can get to and so on, but a guest would not be in those areas, obviously.