@Milkshake
Knowing that the one you truly love getting together with someone else.
And the worst part is, to your best friend.
Komi didn't tell the name of her crush to Manbagi (and she wasn't obliged to do so).
Manbagi never tell anything to Komi about her feelings toward Tadano.
Nobody's at fault.
Manbagi can't blame Komi for sniping Tadano (since Komi like Tadano first).
Manbagi can't blame herself for trying to get in between them (since she didn't know about Komi's feelings).