Once I was playing a mage on Master, and I had an Ancient dragon show up much earlier than it should have level-wise, I think like level 22 or something. Its fly-by breath attack was doing about 90% to me. Basically I ran to the closest city, Falkreath, to try to get the guards to kill it. They all died and it was after me again. I went through Riverwood, Whiterun, a few Giant camps, a few Bandit camps, just leveling my Restoration up like crazy with all the healing I was having to do. The dragon was killing everything and still had like 60% health at this point. Well, I went through Dawnstar (all died) and finally made my way to Winterhold where the powerful mages there were finally able to kill this dang dragon. Yes, it died, and I accidentally fell from the cliff all the way down to the river below, and I died too.
Other than that, Mudcrabs sneak attacked me while I was hunting a deer.
I tried to win a fist fight against Borkul the Beast.
I thought I had my Orc Racial ability in the "shout" slot, so I clicked it to fight Krosis at Shearpoint, and I accidentally Whirlwind Sprinted myself off the cliff to die. That's happened a lot actually, I usually don't die, but I think I have something equipped, then I Whirlwind Sprint instead.