In Spain not much years ago happened a slightly different but related trick. A party changed late their logo in the ballots to a circle. Casually their rivals from the opposite ideology had adopted before a thin circle as a logo in the voting sheets (shared by all parties). The second circle was more thick (and more visible) than the other and had different letters inside. Clearly different for people with a good sight, maybe not so much for elders.

I bet that this didn't helped at the hour of counting the ballots and I wouldn't be surprised to know that a few votes were assigned wrongly that night at the slighest opportunity.