Bad Idea

Many things in NetHack are a Bad Idea, although that often only becomes apparent after your demise, which may well be YASD.

A few bad ideas include :

Casting force bolt while in a shop
Picking up items in shops while afflicted with Teleportitis
Teleporting the riders corpses
Eating "just one more" giant corpse/royal jelly
Strength training by over burdening your self
Wielding a cockatrice corpse while burdened
Walking around while blind, without gloves
Letting that monster hit you just one more time before you heal yourself
Breaking a drawbridge while standing adjacent to it
Having your wand of wishing in your main inventory instead of a bag
Putting a wand of cancellation or a bag of tricks into your bag of holding
Walking around with a cockatrice corpse wielded. Levitation is much safer
Sacrificing at a cross-aligned altar while having bad alignment record
Killing a co-aligned unicorn, breaking a mirror, etc
Picking up non-identified gray stones (identification is done by first kicking and then rubbing the stone on an iron object)
Killing the Wizard of Yendor before collecting seven candles for the invocation ritual
Reading a scroll of genocide while confused

See also