Try deactivating all plugins and check to see if this resolves the problem. If it does, activate the plugins one by one until you find the problematic plugin(s).
Also see:
http://wordpress.org/support/topic/279132
I had the same problem, I did what esmi suggested and isolated the offending plugin… “Flash MP3 Player JW2.3”.
Thanks for posting craig, thanks for the suggestion esmi.
An update, after deactivating “Flash MP3 Player JW2.3 (which sucked because I was using it, and very fond) I still had problems with my editor, the HTML and preview wouldn’t work and I’d get random breaks in my posts. I isolated it to “CDN Tools” plugin.
I also had problems when I updated wordpress with my theme “remix”, which remix quickly resolved to their credit. Still I will be waiting a few months the next time wordpress makes an update, to be sure everyone else catches up.
WordPress is awesome but the simple fact of the matter is that with so much that everyone builds on top of it I think that wordpress needs to make their updates beta or first offer it for people to have time to update their themes and plugins.
