I've stepped on various items laying around the floors of Skyrim that glitched and took quite a hefty chunk out of my health. While they may be lower level enemies, they can take you by surprise when your character screams in pain for no apparent reason, and a bowl or whatever flies across the room.

bears there always sneaking up on me

not to sound like a wuss but the wolves has scared the crap outa me, you can kinda hear them sneaking up on you then they bark really lound and scare me every time

Invisible volkihar vampires..

In my lower levels, I hated spriggans. I remember my first encounter. All of a sudden everything's green, and I'm inexplicably losing health. I was unsettled to say the least.

Indeed, I hope to be one of those soon ( a true one at that )

The sound Chaurus Reapers make really gross me out. I also find most of the Dwemer enemies to not necessarily be terrifying, but certainly creepy in context with the derelict nature of Dwemer ruins.

I found a random dreamora a high level and he came out of. O where and. Ack stabbed me