I actually fell for this one in my first and only run towards true pacifist ending,knowing little to nothing about the game.
After hearing from one of the frogs that sometimes you need to weaken a monster before it can be spared,and finding myself unable to spare Queen fluffybuns right away,I started to painfully lower her HP to halfpoint....
...The very next hit was a killing blow ...
Mistakes were made,i reset....BUT FLOWEY KNEW EVERYTHING.
I actually looked up how to stop Asgore after dying to him the 10th time because I was trying to just wait it out until he gave up, like everyone else. When I found out you /had/ to fight him, it was like... just out of no where. I know they kind of hint that you can't spare him with how he breaks your mercy bar, but I figured that was more so for locking away the 'flee' option.
It felt wrong to attack him, but at least you get to spare him after you beat him... Unlike Toriel.
I spammed spare first time i played the game because I really didnt want to kill her. When her face and attacks started to change did i actually realise i was doing the right thing which was pretty lucky.