Wasn't Goku just a False Super Saiyan in the Lord Slug movie though:
http://dragonball.wikia.com/wiki/False_Super_Saiyan "One way to put it is that the False Super Saiyan transformation occurs when a Saiyan has the required physical power but lacks the required mental strife to transform into a Super Saiyan. It is apparent, however, that the form gives a large power increase, enough for Goku, who was being easily beaten previously by Slug, to now quickly overwhelm him." I'm not sure how credible Dragonball Wikia is, but I tend to agree with it.