Every time I quote something to reply to it (“reply with quote”), I’ve been getting this contradictory and nonsensical message. WTF? 
You can multi-quote across threads. Perhaps you accidentally clicked on the multi-quote button somewhere along the line, and that quote is still selected.
Yep, this must be it. To fix it, just go ahead and quote – you can delete that first quote (which probably isn’t relevant to whatever you’re posting) manually.
Much better than my method, which would have been to tell you to clear your cookies.
The boring way is to click on the link that says “deselect this post” at the end of the message that tells you you have selected a post that is not part of this thread. 
Gah. I do it so rarely I forgot about that link! :smack:
But much worse than my method, which would have been to tell you to relax and bake some cookies.