There was a Crow who was caught in a trap and prayed to Apollo to release him, by making a promise to offer some incense at his place.
But when he was rescued from his danger, he forgot his promise.
Shortly afterwards, again caught in a trap, he passed by Apollo and made the same promise to offer incense to Mercury.
Mercury soon appeared and said to him, "O you base fellow? how can I believe you, who hast disowned and wronged the former patron?'