A user mistakenly posted in the wrong category in our forums as he had read-only access to the category he wanted to post in, but he swore that when he navigated to the category and subcategory, the New Topic button displayed as if he was allowed to post there. He was not, but the New Topic button still displayed that way.
I created an alternate account and gave it the same trust level as him, and confirmed that the issue occurs when you add a tag. The screenshots below can probably explain better:
Notice that when I add the “exploit” tag, it appears as though I can post create a new thread.