LOL Jim, don't you hate it when that happens? I've done it more times than I care to admit. FWIW last time I did it, I saw it immediately after posting and was able to use the Back button in my browser, then copy and paste the original message back into the original poster's message. It still leaves the "Edited by" tag at the bottom though, but at least the name of the poster and their message are intact.
In case anyone is wondering why we occasionally have this slip of the keyboard, attached is a photo of the adjacent Quote and Modify buttons on the staff 'view message' screen.
That reminds me - it's on my fix-it list.