During a game of Commander at my local store last week some of the folks I was playing with stated that conceding the game was a 'sorcery speed' action. The playgroup made some valid points about this preventing collusion and also the somewhat-irritating situation where one player takes control of someone else's creature and swings for lethal damage with it, only to have that creature's owner leave the game, taking the creature with them.
At the time I just accepted that this could well be the case and got on with the game (I had no intention of conceding) but in a moment of curiosity decided to do a quick google search which brought me to the below entry from the comprehensive rules:
104.3a A player can concede the game at any time. A player who concedes loses the game immediately.
This makes it pretty clear to me that by normal rules a player can concede at any time, they don't even need to have priority.
Does anyone know whether this supposed sorcery speed concession is just a myth/common misconception or is it some sort of rule enforced only in multiplayer tournaments?