If you’re getthing this error, it’s probably because you have a /stable directory that got created while you were trying to install the plugin. So check that you only have /vendor/plugins/attribute_fu and not vendor/plugins/stable.
Also, if you want to install attribute_fu but using piston doesn’t work, try this:
svn checkout http://svn.jamesgolick.com/attribute_fu/tags/stable vendor/plugins/attribute_fu
Happy nesting!
If you have any more info on this, it would be appreciated. Since I still get that error, but do not have the stable folder in my plugins directory