Post your most annoying, but not fatal mistakes here. I'll go first:

1. A wizard trying to pass the animated forest armed with a poor first hand weapon, teleportitis and no teleport control. Urgh.
I had just lost teleport control and gained teleportitis from sipping pools, hoping for (and getting) a wish, and was on my way down to Khelly with an AoLS. Figured I didn't want to risk the Dwarven Halls, so the Animated Forest was the only other choice as I had access to a few invisibility potions. I realised my mistake quickly, when my teleport hiccup threw me halfway across the room but in the wrong direction... It must have taken me more than half an hour to get out! Next time I'll ID scum for a blink dog corpse instead. Way quicker.