I had a plugin installed in WordPress. And had it activated. Then I got an error message like „Fatal error: Undefined class constant ‚plugin_version‘ in /www/htdocs/w00fb338/wp-content/plugins/share-on-diaspora/admin.php on line 17“, whenever I went to any admin or authoring functionality of my blog. Just reading it seemed to work fine. So there was no way to go to the plugins section of my dashboard to activate or uninstall that plugin, because there was not way to get there.
Since this blog is hosted on some typical web hosting service, there is no ssh access, which would be helpful, because there are command line interfaces to manage a wordpress installation. What remained was ftps to access the file system. Then the plugins could be found in the directory
and the plugin creating problems could be deleted using file operations.
I think we agree that this is kind of a hack, but it worked. The wordpress installation detected that the plugin was missing and deactivated it. Then it was usable again. I assume that this applies for any plugin. It should of course be used with care.