This one happened to me yesterday, and it was quite noobish. I was pushing mid lane on my own, when their fed AD katarina jumps on me and ults, so I proceed to exhaust her to be able to fight back/escape depending on how it goes... thing is, I mess up with the buttons and end up teleporting to a minion next to her... not to say that in the mean time she shred me into pieces.
Though my Noobiest death must be back when I was level 5 or so, and after a winning streak(can't imagine how) with sivir, join another game, thinking i must be so pro at this game!. So I go bottom lane, and happily go check who is in the other tower. Suddenly pantheon and some other guy jump on me from the bushes(that I should have thought that they were for decoration only) and get totally owned. So i get back angry to show them who's the boss... throw the BB and get into the bushes(yet again) to get equally owned... I think I spent a week running away from any bush in the game!