People circling the name of their preferred candidate instead of a cross in the box, striking out the name of other candidates as an indication of who they want to vote for etc.
If you think about it, how would you know if you've not done it before? Even if you notice where it says (in very small letters) to put a cross in the box, how do you know that it means the little box to the right of your chosen candidate's name and not the big box in which the name is printed?
I forget the actual wording on the ballot, but I think it says to put your cross in the box 'next to' the name of the candidate. Therefore, you could even work out that the little box to the right of candidate A (your choice) goes together with the big box with their name in it, so you mark the box NEXT to it i.e. the little box for candidate B.
It says to mark one box only, but if people don't know how it works, they could think that, instead of choosing one, you have to vote to 'disqualify' one by 'crossing out their name'.
If you think of something as basic as a wordsearch, even there, some people will cross through the words that they find, some will circle them, some will colour in the individual boxes making up the word, some might even black out the letters that don't make up words.