Strange little quirk, but it just did it again - "You have posts selected that are not part of this thread - Quote them or discard them?"
(Using Chrome 10.0.648.151 today, but it's done it to me in Firefox 3.x too.
Shouldn't those be cleared after you've completed the Reply and posted it? Seems simple enough to put that in the Posting script, last step before it throws you back to the forums and displays your minty-fresh message.
Someone ask the VBulletin support desk about this one... Unless they're lurking...
--<< Bruce >>--