A Horrible Mistake is a silver bullet card meant to destroy decks based upon copying one card, like Gib Kcir (16/RR-chase), Necba the Wrathmaker (14/DU-chase), or Highmaster Illithios (21/DU-chase). However, this card has many of the problems associated with most silver bullet type cards; it's nearly useless when the target isn't there.

A Horrible Mistake can kill any card, and then kill what it was copying. Now this can be very useful, and even kill three cards when Gib Drawsemaj (22/NS-chase) is the target (but only when it is first played, because then it stops copying after that). However, when the card being copied is yours (like with Land of Legends (4/IQ), but not The Shadowlands (2/IQ), since that steals), you can have your card dumped just like your opponent's.

Either way, those cards go to the Abyss, and the ability to force the opponent to send two unknown cards from their deck (if you catch them copying their own card) is very potent.

Many decks carry some card that copies another, but the risk of having a dead card in your deck is quite dangerous, as every card needs to count in a game. Some people will decide this card is worth that risk, for the potential card advantage against a single opponent, but many won't.

A player who intends to copy other cards must be aware of this card, and figure out a good way to stop it (champions immune to events, event stoppers, etc), because it could destroy your theme.

If you don't intend to play such a theme, only carry this if you've been hurt by said theme often. Otherwise, a more generally useful card would serve your deck better.