Today the British Supreme Court held (by an 8-3 vote) that Parliament must approve the invocation of Article 50 for the country to leave the European Union. I think this conclusion is correct, but the decision does open up at least one can of worms.
The House of Commons will almost certainly approve such a bill, but what about the House of Lords? The Lords still have the power to delay the enactment of any legislation passed by the Commons for up to one year. If they do that (and they well might, given that the Lords are far more pro-EU than everyone else), then that would delay the start of the exit process until 2018. I wonder what elected representatives and voters will think about that if it happens.