When editing a post in WordPress 3.1 recently, I found that the visual editor was missing the formatting buttons which usually appear at the top of the editor and the text was invisible (white on white) like this:
After some investigation, it seems that the problem was caused by the Scissors Continued plugin which I had recently installed. When I disabled the plugin, the problem was solved:
I have not investigated to find out what the actual problem was, although I could see that the buttons initially loaded then the screen reformats and the buttons are hidden. I have not yet tried the functionality of the Scissors Continued plugin and, although the features sound very useful, I will not be reinstalling it if it means losing formatting options when editing posts.
I hope this helps you if you hit the same problem…