They could, and they might. It would depend on how serious the mistake was, and how relevant to the whole application. Contact the UKVI as quickly as possible and see if you can straighten things out with them directly.
ALMOST IMPOSSIBLE. BUT YOU CAN TRY.Go to the Canadian embassy and make a request. Just make sure your story touches the consular’s heart. Remember if you are not successful, you might never be granted a Canadian visa in future.