Couldn't for my life get why I couldn't assign thumbnails to my posts, add tags to custom post types and other mysterious glitches. If i added tags, they we're saved, but killed immediately whenever I tried to edit the post again.
Then I turned off the Visual Editor Custom Buttons, and everything started to work again.
I seems the VECB with its use of custom fields interferes with other metadata somehow.
A pity really, since such a plugin would be really good to have.