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The misunderstanding regarding the Dragon arose because its ban wasn't specified in the rules. I thought it was obvious, but it turned out not everyone shared that view.
In my opinion, the most appropriate solution in such a case is to nullify the pick, as the player chose the Tavern expecting the Dragon to be available. A similar situation arose later in another match, and the pick was also nullified.
Both players agreed with this decision at the time, so I consider the incident resolved.
Arguing in ICQ isn't ideal, but it's a non-game-related issue, so it wouldn't be appropriate to base any organizational conclusions on it.