World of Warcraft. Too many times, I found myself in a raid, doing well against a not-easy boss, and we get it down to less than 5% health. The raid tunnel-visions, we forget some mechanic or another, annnnd wipe.
This has actually happened a handful of times when running old raids solo. Big pull, run to a boss, get stunned or something, inevitable death. Also, Icecrown Citadel has the Gunship encounter, and I don't care how hardcore you think you are, it takes every ounce of gaming skill you have to survive it alone.
Dead Space 3. I've got access to some seriously supreme weaponry. I still get murdered more than I'd care to admit. I dunno, maybe it's just because I try to stay too calm, while let's players always spaz out whenever more than two enemies are heading towards them.
Hearthstone arena runs. I watch Totalbiscuit all the time, I know this game inside and out! *gets utterly humiliated* Welp, that's gonna happen. Time to not play this game forever!
Skyrim. I handled just about everything that came just fine. It wasn't really super easy, but I got pretty confident. Dragons eventually became easy enough, with a handful of potions. But then I ran into some magic users, and... well, I learned that magic resist potions are a Dovahkiin's best friend.