In the fight with Toriel, selecting the attack command and then not attacking
at all (as in you just completely miss the meter) has her act just like you SPARED her. Which meant that when Asgore destroyed the MERCY button, I thought I had to start purposely missing attacks. [I remember even thinking that that would be one more link between Toriel and Asgore, on top of the identical house layout and the reprise of 'Heartache' during the battle music, and got really excited
]
Which lead to one or two hours being completely lost.
Also, Undyne's date sequence tells you to just "talk with Asgore". And while talking does bring up "do something else" prompt, a huge part of the game (well, the pacifist run) is how
determined you are to not fight! So I thought that the game was trying to fake me out!
I ended up reading through Toby's twitter feed and finding out that yeah, you're supposed to actually attack him. I have no qualms with labeling that part of the game at least slightly unintuitive.