when you fail to curtsy a royal blood...

Well, if you ever had the chance to meet them, you won't be exactly forced to curtsy or bow before a Royal but it will be considered rude. Most especially, if you are in England where you are expected to follow their traditions and the Royal protocol. You won't be bashed publicly but you will the subject of gossip mongers and other attendants in the event. In a sense, you are a persona non grata (an unwelcome person), a person who is rude to the country you are staying in. As the Royals (The Queen, in particular) is the embodiment of the British nation, it will look like you are being rude to the whole of UK. For your second scenario, you will never mistake a Royal for a stranger. Partly becuase their faces are plastered everywhere in the media. Also, in an event, if a Royal wants to speak with you, the help will carry out the orders. After that, you are expected to follow the help to the direction of the Royal. If you did speak rudely to them , I would guess that they will be quiet and keep their thoughts to themselves in a public setting. Just don't expect to be invited again.