It *might* be considered an error note. There's a thin plastic film that covers the holographic strip on upgraded Journey series notes. The 20 and the Coat of Arms, as well as some background tint, are printed onto the plastic. When the plastic strip is removed, the design printed on it will be missing from the note. I once found a $5 note which has the plastic film belonging to another note stuck loosely to it...
{http://www.give-a-buck.com/special/plasticstrip.jpg}
Because the film can come off after the note is printed, it should not be considered an error by definition. It would be impossible to prove how and when the film came off if you found a note in circulation missing part of the counter and the coat of Arms, so I don't think such a note would carry a premium.
Qcumber's note with half of the strip missing is interesting, but ultimately the same thing as a note missing the entire strip.