On one of my earlier character I went up to a giant camp, not knowing they considered it sacred, two giants were there and when they saw me I gas far as (From whiterun hold) to windhelm! I wish I knew they hated water back then, could have ended a lot sooner!
Also when I was first trying the college of winterhold I fell in to the sea of ghosts from the walkway

Well, nothing compares to fighting that giant Slaughterfish in Oblivion, but Chaurus generally scare me.