I seem to have the same issue. I just registered and posted a new thread after which I got the message:
"Forum topic Would you get a used high-end plasma or budget 4k TV? has been created."
However when I checked the forum where I posted it ( http://www.whathifi.com/forums/tvs-and-projectors ) there is no such new thread. Noticed I had some kind of activation email in the inbox, so I clicked the link and logged in again. Tried re-creating the thread but the same thing happened. I.e. got the confirmation but no thread is visible.
Is there anything I can do about this? Being slightly wiser I emailed myself the contents of the post so that I can try again later (or more probable use another forum if this one isn't working at all).
I really wish you would inform the user in some way if you are going to delete their first threads as this is of course quite frustrating. Of course if you just moderate the new users posts for spam, them perhaps a different message would be in order.