What happened was that you deleted it, then tried to delete it again and the software gives a misleading error message.
It can't delete it as it has already been deleted and gives the message that you can only delete your own posts.
Then when you looked for the deleted thread it was not found and it gave the message saying "The topic or post you requested does not exist or has been moved to a restricted area". In your case it was the does not exist part, as you'd deleted it.
e.g.
http://www.able2know.com/forums/viewtopic.php?t=999999999999999
That thread doesn't exist, and will give the same message.
Basically, it's just giving an error message that isn't specific enough in the first instance and in the second it gave two options, one of which was the correct one.