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.
You're underestimating me... I can easily do it if I click "+Merit" on the wrong post - I get confused quite often here because "Report to moderator" and "+Merit" are my two most-often used features and they're on the opposite ends of a post - and then don't pay close attention to the username on the next page.
Merit sending should happen on the same page. Or at the very least the page where you enter the merit amount should display the post content that I'm meriting. There's definitely enough space on that page. And the damn zero default should be removed - I can't think of any good reason to have it there.