User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130082822

(5)

Cooler's Revenge is canon. Now, each canon disturbance will be discussed: This was most likely done so that Cooler's Armored Squadron could recognize a Saiyan (without his tail, he'd look like a normal human), and it's not out of the range of possibility that it also re-grew on its own. Even if not, this minor aesthetic is not enough for canon breaking material. This is true, and may be a canon breaking element, however it could have just as easily not have been. We don't know if Goku still thought he could win, in his normal form. He might of not wanted to transform, for unknown reasons. Regardless, in the end he did transform, and no canon was broken. This movie is also one of the few that can fit into the canon timeline without any confusion, therefore helping sway the ruling.
 * Gohan's Tail
 * Goku was unable to transform to Super Saiyan, until angered.

Final Ruling: Canon