You're biggest letdowns in desktop dungeons, here goes mine:
Having an awesome run in western jungle as a berserker, doing complicated task pt 2. I spent so much on preps I have so much gold with me, using it to just buy items from stores and converting those items. Converting every glyph in sight. Kill everything besides lower level monsters at level 8, the boss and one level 9 monster left. I'm worshipping jehora so I plan to chaos avatar on level 9, I have 100 piety. I mindlessly start clicking on all the lower level monsters, end up at half health eventually, and get killed by one of the monster's death gaze.