As I understood the story (the photo of the ballot that is sometimes circulated is just one taken from Twitter), the ballot had Brexit written all over it, and an arrow pointing at the Tory candidate's name, so ended up counting it as a Tory vote because of that.

That's what candidates do I recall a talk by a Labour Candidate where in his election arguing that a ballot with I vote for that bastard was for him.

This was a constituency where the previous incumbent had had some problems with his younger GF