So I hit the Back key, copied the whole thing -- what I had selected from the original post along with my reply -- and pasted the whole thing back into the window, and it came out fine.
This does not, as I originally thought, have anything to do with punctuation. There are here, for example, three periods after "it", but there are also two and a comma after "Btw" which post fine. It's not the result of a choice or of a selection. It's a software glitch. This has never happened to me before on any other website, including those who use similar software.
This is how the post ended up:
http://www.elitetrader.com/vb/showpost.php?p=3940841&postcount=13
All this took place within ET. Neither Word nor Notepad nor any other editor was involved. Nor was it copied and pasted from an external source. Just Quote, respond, and Submit Reply.