I would suggest an intermedial page where you are sent when you click the link to (un)subscribe with the following:
You are about to UNSUBSCRIBE form this thread.
Are you sure you want to proceed?
Accept | Cancel | Back
Accepting would bring you to the "The selected thread has been removed from your subscriptions list" ("The selected thread has been added to your subscriptions list") page.
Cancelling would show something like "No changes were made in your subscriptions list".
Back or Abort would bring one back to the thread/forum.
This way it would be even clearer and you'd avoid having to ask MySQL every time a thread is visited
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