Thread response window switches to overstrike mode

I noticed a couple of times this morning, if I'm composing a thread response and try to edit: sometimes it sort-of switches to overstrike mode. As I type new text, the existing text to the right is deleted instead of shifting.

 

No, unfortunately: insert key doesn't seem to be the problem.

And as far as I know, I hadn't accidently hit insert to begin with. This happened a couple of times this morning, an intermittent thing. I did try hitting insert key to undo it: the site's edit window stayed in overstrike mode regardless.