Found this while verifying JBIDE-10615 .
When you add a seam facet to any project you're shown a warning: Further configuration is required. By clicking it you open a dialog where you're asked for the seam runtime, but the dialog allows you to press OK even if you don't add any seam runtime. This results in an error (see screenshot).
This is the screencast of the scenario:
http://screencast.com/t/kuL9WD61P