You may get this occasionally if you follow a link from a post or from an email if you are using the auto reply notification. The first one will be rare but the second one could be more common. This means the post or topic you are looking for has been removed to a forum you do not have permission to view. there is a few hidden forums. For example the testing forum is not public so if you were logged out and tried to access a topic there you'd get that message. Same thing with "Flame suit Central", a member that has joined that group may inadvertently post link to a topic there and you won't be able to acces it if you are not a member.
We also have garbage can where all "deleted" posts go and this is where most of these links will come from. Nope, we don't delete anything here. If a spammer posts a reply to a topic you are participating in and you are using the reply notification then you'll get an email as usual. In the meantime we will have swooped from above to kill this crap and move it to the trash. Since you cannot view this forum you'll be denied access to it.