A pal of my own ended up being confused by the Bloodthirst ability: "Bloodthirst N - If an adversary had been dealt damage this turn, this permanent comes into the battlefield with N +1/+1 counters about it."
It's apparent to any veteran secret player that "opponent" means a person who is perhaps not you (or a teammate of yours, in a multiplayer format). For example, you will find cards that say things like "Target opponent discards two cards" or "Each adversary comes back a creature she or he controls to its proprietors hand." There are no cards that state such things as, "Each opponent gets -1/-1 until end of turn."
But my buddy believed that an "opponent" could imply a player or a creature managed by an opposing player. It is a totally reasonable reading of English clause "If an adversary was dealt harm" - you performed deal injury to a thing that just isn't on your side! - but it obviously misses the whole point for the Bloodthirst auto mechanic.
Another issue of confusion was whether you drop the game for having no cards left in your collection. You don't; you drop the video game when you've got to draw a card and should not because there are not any cards in your library.