Brexit.
But this will be a chronic bitch. There is really nothing quick about it at all.
Will UK citizens living elsewhere in Europe have to go home immediately?
Oh no. First of all, the UK will have to invoke paragraph 50, which is essentially about saying "we want to leave". There will be a period of a couple of years when the UK will negotiate the terms of their leaving the union and most likely another few years when they will actually try to implement those terms. During all of that, they will remain a member country and will have to follow any treaties in place.
It is going to be messy, though. and I think the UK will go through a recession starting just about now. I hope I'm wrong but I don't think I am.
The UK will also have to negotiate new agreements with just about everybody. Farage et al think this won't be a problem, but I suspect it will be a world of pain for everyone involved. I'm happy to be wrong but I don't think I will. I think this is all going to hurt, and a lot.