I use the google groups interface on my desktop and the iPhone. In this case I was using the desktop browser and clicked on the big red "POST REPLY" button at the top of the page. I guess to play nice with those who use the discussion group through their email I should instead click on the specific "Post reply" button on the last comment in the thread?
Another thing about the google groups interface on the iPhone. Once a thread has been started, everything in it is one long chain and there is no differentiation of split threads or who replied to who.
Lastly in this off topic post, is there any way to by default not have every reply contain the quoted text of every reply before it? That may be ok for a few back and forth emails, but in a long discussion board its really bulky and confusing to read.