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.
Mistakes come with volume 
Up to last Friday, I've sent Merit 7381 times. I usually open the Merit link in the background, then go through a bunch of them at once. I remember once Meriting the wrong post (which wasn't so bad anyway) because of using too many tabs at once. And there's this