blundering in a winning position (specially when it's a missclick, or when the blundered piece is a major piece), I don't really care on what happens if I'm lost, but throw the win away is always painful
Sound strange but I don't like winning against players way higher than me
Misclicking when you were near winning
it's spending the time to figure out a long winning combination and when you're almost there... you missed one move and all is lost.