This question already has an answer here:
I'd experienced a bug in Flag Window.
Browser : Google Chrome (Version 41.0.2272.101 (64-bit)), Firefox (Version 36.0.4)
OS : Ubuntu 14.04 64-bit
In flag window last option other (needs ♦ moderator attention) containing a Text field to write details about flag. It goes out of Flag window.
It occurs in normal browsing as well as in private browsing/incognito mode.