This happened to me as well. Turns out I was using a theme with a custom comment form (WP-Bootstrap). It was missing a few elements it needed to create the panel above the textarea, and was erroring there.
Specifically, it errored: http://i.imgur.com/FnabJBz.png
Ran into this page when trying to debug this effect on another website I was scripting for. I noticed removing wmode=transparent param tag from the object stopped this effect.
