After updating a post via EDIT (changed one word), the post lost all formatting and showed HTML-like coding. I backed up the topic post's content (it was very large) and put up a message stating that there is a problem.
I already reported this. I guess it's not the quick reply function, its the editing function. Apparently any HTML gets put back into the post for some reason, but since HTML is turned off, it gets outputted into the final post looking like code, rather than being parsed properly.