Republican Party rules say that on the first ballot, at the national convention’s vote for President, delegates must vote for the candidate they said they would vote for when they were elected. But, according to this article at FrontloadingHQ, if a delegate disobeys, his or her vote still counts.
I disagree with FHQ’s interpretation.
At best it would count the same as a spoiled ballot, which would show up in a popular election as an undervote. Whether it would count as an abstention at the convention, and whether this had an impact on the result would be another issue.
I think that there are also state party rules that treat an attempt to vote for another candidate as an automatic resignation, so that the secretary would never be aware of the misvote.