Do you have a close friend or loved one who also visits TFP? Do you ever share your computer with them? Do you keep posting under the wrong username, because they forgot to log out and you didn't check before hitting the Reply button?
I've been seeing this more and more frequently these days. There is a very easy solution. Switch your theme.
User CP / Edit Options / Miscellaneous Options / Forum Skin
There's a bunch to choose from. Talk it over with your lover/buddy/whatever, and make sure you have a different forum skin. If the colors aren't right, don't post!
This has been a public service announcement from your friendly obsessive librarian, who is troubled when things get misfiled.