Apparently it's a feature, and I discovered it the exact same way. Tried to enter a thread I made, and oops... the thread title got edited something odd.
I guess it's a feature for thread-creators to edit the thread title on the fly?
It's especially annoying for me since I can edit all thread titles. I have a habit of double clicking in the window so I can scroll with the arrow keys, but half the time I end up selecting a thread title and have to click out before accidently changing it to jibberish.