The only especially dumb mistakes I've made involves approaching enemies with not enough caution, before knowing how strong they really were.

Once was by wandering into the mountains while I was still in basic leather. This was very early into my experience, before I had figured out that every biome corresponded to a difficulty level. At the time I assumed the stat-climb of enemies and equipment would be far less steep across the board. After all how could a single wolf be more of a threat than a troll with a log, right?

The second dumb mistake was made for much a similar reason. I approached a 2-star javelin fuling while I was still in iron. Now I could take a fair beating from a towering elite draugr. Bonemass didn't make me break a sweat. And even those wolves that could once obliterate me were starting to become manageable. Surely this scrawny, pint-sized fella with a pointed stick couldn't hold a candle to any of those... could it? So I went in for the sneak attack, and that was when I began an hour-long prep to retrieve my best gear back.