RE: Flip a coin, if tails...Um...oh. crud.
Ancient Mew has only one attack. I made the effort to decode it myself (WITHOUT using the key; when you confirm where they say, "Mew," "Weakness," and "Resistance," everything else falls into place).
I found out that they were legal for a short time, provided that you use opaque card sleeves. I don't think anybody actually used them though.
That Rocket's Minefield Gym...I want that card, just to mess with people. There's a similar logic error in one of the rulebooks, though I forget which: It explains that when you're facing a Baby Pokémon with the Baby Rule, you have to flip a coin, but it doesn't explain what you're supposed to do after that--just that you flip a coin.