But how do you even get it wrong? A separate tab opens when you click "merit" and you get to enter how many merits you want to send to that post. I just couldn't think how you'll commit two mistakes in a row.
Aside from what LoyceV said, few people uses script which allow them to merit a post only with 1 button. But usually it's more common to send wrong amount of merit (e.g. 1 to 10/11) rather than merit to wrong post.