We have a similar problem. I hadn't tried it on a deleted post, but it does the same thing with a thread that is moved. For example, we have an invisible forum where we stick spam, flames, etc. We moved a thread into it, but the forum it originated in still had the "Last post by" link to the thread. Even worse though, if you clicked the "Last post by" link, you could still get the thread, and reply to it, even though it was now in a forum you didn't have access to. <img src="/forum/images/icons/crazy.gif" alt="" />
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[:"red"]Lisa[/]