Surviving a hit from anything 30' or taller.
In Crystalis, the classic old NES game no one's ever heard of, the end boss. Is. Too. Easy. Just, ugh. Most anticlimatic boss ever.
In fighting games, when they do fatal things (like neck snaps) that 'only' do a whole bunch of damage. Or, in fighting games, when the 80 pound 12 year old girl character can hit the 350 pound huge-guy with a jab and send him four feet into the air, vertically.
Devil May Cry 3, the fight vs Lady. You're capable of catching bullets in your teeth, and this regular human is capable of outrunning you? And when you finally get her with the sword, the magical sword with the strength to use a motorcycle as an effective bludgeoning weapon... And it 'only' hurts a bunch instead of bisecting her? Wth?
Most hit point / life bar systems in general, really. Make it hard to hit them, not easy to hit them and hard to kill them. THAT is how things survive in real life.