Showing your opponent cards when you're not required to.

[Deathjester]

Suppose a player in a tourney duel makes the mistake of showing the card(s) moved from the hand to the Deck with Good Goblin Housekeeping or Magical Mallet. Would the duel still be able to continue or would it be "damaged beyond repair"?

 

[DaGuyWitBluGlasses]

Magical Mallet, its not much of an issue and a minor warning should do.

And although some cards might make an issue of Good Goblin Housekeeping.. (some card involve the bottom of the deck or flipping the deck over) those cards are seldom used.

However, in other card games (bridge for one) revealing a card by "accident" is often a way of cheating, so you might have more to look out for.