Usually that shows up when you've clicked on a locked or deleted thread.

 

Me neither. Was going to the 2nd page of a thread and this shows up.

 

QC is close...it's probably a thread that's been deleted/moved. Locked threads, you can still see but not post on.

 

I think your correct on the closure. Went back and looked at that particular thread and it was showing 0 posts. Strange that it wouldn't let me access to read it though. First time I've seen that. Thanks COC.