KIngdom Hearts Twofer here.
KH1, I manage to get all the way up to facing Darkside on the Destiny Islands (in the early part of the game). I'm doing good, but those stupid Shadows deal more damage than they drop HP balls, which sucked royally for me, being a novice KH player at the time(since it was my first playthrough, I didn't know of this specific boss, and wasted a Potion during a sparring match with Riku). So, I get close to winning, morbidly low on HP, like 1-2% left, and Boom! Out of nowhere, a Shadow nails me. I die. I figure, no big, I'll just redo it. Nuh-uh. My house's power dies. This moment of terror is compounded by my realization that I never saved.
Second incident. KH2, Demyx boss fight. I have him down to his last bar of HP, with only a third left on said bar. I have numerous healing items and Ethers and stuff, so I figure 'no big deal, I got this shit flushed'. Then he does his Water Clone timed minigame. 15 seconds to get rid of 10 clones. Simple, right?
WRONG. I have a second left, and the last guy is ij front of me. I'm just about to land the finishing hit...And the timer rings, meaning I have to redo the entire fighf again.