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1.) install the latest media module --dev version and enable it as usual
2.) do any futher steps like provided to use its functionality as usual
3.) Create or use an existing content type and create another image field using the media file widget
4.) (Not sure if it matters) create a node of that content type (that's at least what I did)
5.) Disable media module.
6.) Navigate to your content type edit fields form. The error should be ok here for now, saying that a field can't be shown because of a missing widget. Well actually it isn't ok, but this is another issue how to deal with disabled modules in general.
7.) But now comes the funny part for us on this issue: Re-enable the media module and navigate back to your content type edit fields form again. E Viola, The error will persist. And ... the field still will not show no more. So the field will be hidden for all times, since you even can't delete it. The only chance you have is, to delete the whole content type, which will lead to a warning that all content will be lost of course. Not good.
To demonstrate the errors obscurity I added another image field using the media file widget again while the error that the widget is missing and a field is still hidden persists :-)
|Multimedia File | Drupal-7.12 Teststage Linux 2012-02-27 06-33-35.png||100.59 KB||diqidoq|