Here in the UK you can ask for a replacement ballot paper if you make a mistake. So you vote for candidate A, photograph it, swap it for a replacement ballot,

Here in the UK you can ask for a replacement ballot paper if you make a mistake. So you vote for candidate A, photograph it, swap it for a replacement ballot, vote for B, photograph it, etc. and get paid by all the candidates. Except the candidates know that a photo of a ballot paper proves nothing.