Is there any [consequence] for someone who forbade a person from throwing stones at street light poles, considering it a detestable harm, then sought forgiveness and retracted his fatwa?

If you see someone throwing stones at lampposts and you stop them, your action is correct. This is because such an act is an act of corruption and an assault on public interests, causing harm to people. Your prohibition of it falls under the category of enjoining what is right and forbidding what is wrong, which is required of every Muslim, due to the Prophet (peace and blessings be upon him) saying: "If you insist on sitting in the pathways, then give the pathway its due right." They asked, "What is the due right of the pathway, O Messenger of Allah?" He said, "Lowering the gaze, refraining from causing harm, returning greetings, enjoining what is right, and forbidding what is wrong."