Another classic boss scenario for me from Bloodborne/Dark Souls is this:
I'm just about to beat the boss and it's health is down to just one hit left, when it hits me, I get knocked down, and I believe I'm dead so I drop the controller in frustration. Then my character gets up and I realize, oh shit - I've got like 1 HP left so I could either heal myself or hit the boss and kill it!
... Only to realize this half a second too late, and it kills me. Fun times.