Human instinct and social conventions already solve the "clumsy defector" problem. The clumsy defector is required to sacrifice any benefit he gained from the defection. Usually this is through the small-loss-of-status of publicly acknowledging their error. But in the game, they must cooperate while you defect.

This restores trust and removes any incentive for the clumsy-defector to do what they did. If they refuse to accept this, then they aren't actually seeking to form a mutually-beneficial cooperative relationship anyway, and thus they must be punished by defection unless or until they accept the penalty by cooperating while receiving a defection in return.