It's sort of sneaky for Team Rocket to do yet another thing to a card. This time it's on their "Rocket's Minefield Gym" card.
So... I suppose when my opponent plays a Pokémon, I can just about put any number of damage counters on it. "Hey, a Chansey! [flips coin, tails] Ok, I'll put... oh... 12 damage counters on it. Hey look, it's knocked out! I get to draw a prize!"
Actually, the missing number there is "2". If tails, put TWO damage counters on that Pokémon.