I don’t know about in the US, but here in Canada, they have printed instructions, both in the ballot box, and on large wall posters, that tell you the method for casting a ballot, typically it, put an X in the circle beside the name of the candidate you are voting for. The posters are printed typically in English and French, but in other areas with large populations that speak another language, they can be printed in those languages as well. For example my area has a large Jewish population, so posters can be seen in Hebrew as well as English and French.
Has the voter, who voted incorrectly been disenfranchised, I mean, that could be intentional, it could be that the person voting is an idiot, or it could be because the voting process is too complex... Considering that most people can operate an ATM these days, perhaps it’s time for all electronic voting...... Then again, for all electronic voting, you need some ability to make sure that the process is correctly counting votes. It would be easy to rig the results.
If you still do it wrong, then your ballot gets tossed. Considering that ALL parties that offer a candidate in a particular voting area, have the right to have a representative present at the counting, and that all ballots have serial numbers, and all ballots cast have those numbers recorded, it works pretty well.