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As discussed and decided at the meeting in Lyon (fall 2012), the application should be required to provide an error callback handler. So the word "optional" in front of the error callback should be removed from the definition of "getUserMedia".
This is exactly why you should be using futures by the way. So that these kind of decisions are not made on a per WG, per API basis.
This is fixed in the April 30 2013 Editor's draft.