It's extremely rare. I've only seem it once. It might come from a black hole award deep into the game but really no idea.

The rules card gives a vague hint. Warp ramps, completing missions, and Black Hole mystery award.

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It's definitely not warp ramp. Reaching Warp 4ish (reflexs) awards an extra ball, 9.0 gives double scoring for a timed period and 9.9 gives an add a ball. I've been round twice and it hasn't awarded a special.

I've got to EA (completing all Lv1 & Lv2 missions) and also not been awarded a special.

Which to me, leaves only a mystery award from Black Hole, completing all Lv1, Lv2 & Lv3 missions or possibly a certain number of Vengeance missions (but that would have to be at least 4)

Seems most likely that it is a mystery award.
Reason it's so rare could be that the mystery awards tend to be semi intelligent and award you progress depending on what you're doing at the time (extra time if in mission, add a ball if in multiball, etc.). So to get a special you'd have to not be playing a mission, or multiball, and also probably maxed out bonus multipliers and pop bumpers before it'd even consider awarding it.

Best just to forget about it!