Unwinnable

While any given game of NetHack may be very difficult to complete, it is generally understood that every game is winnable — that is, no game is created that the player cannot win.

As of NetHack 5.0.0, every game should be winnable -- any unwinnable states remaining are bugs. In prior versions, a game could become unwinnable by design if the hero was unable to begin or proceed on the Quest; other unwinnable conditions were the result of bugs which have been fixed in the current version. The known unwinnable states in previous versions are:

Quest failure (3.1.0 to 3.6.7)

Every character must complete the Quest to obtain the Bell of Opening, needed to perform the Invocation. There were several ways that this could become impossible:

• (3.2.0 to 3.6.7 only) Before getting permission from the quest leader or proceeding down afterward, convert your alignment at an altar. You can no longer receive permission to begin the Quest, and a helm of opposite alignment will not fool the Quest leader.
  • NetHack 3.1 does not check for change of alignment, only for whether you're piously aligned.
• Before permission, the Quest leader dies or levelports down. Due to a bug, a Quest leader revived from a corpse or statue cannot grant access to the Quest.
• Before permission, getting expelled by angering the quest leader or failing the alignment test seven times. The Quest portal is removed, but with the Eye of the Aethiopica it's possible to access the Quest branch, pacify the Quest leader, and seek permission again. Not every character can get access to the Eye, however.
• With an Invocation item or the Amulet of Yendor remaining in the quest dungeon branch, get expelled. Again, the Eye is the only way to access the Quest in this case.
• After permission, but with an Invocation item or the Amulet below the Quest home, convert alignment away from neutral. Even with permission, you cannot proceed below the home level if you're not of your original alignment. Unlike the case of receiving permission, a helm of opposite alignment can fool the mysterious force; however, the helm cannot change your alignment to neutral.

As of NetHack 5.0.0, the Quest portal is never removed, and it's possible to proceed by killing the Quest leader, so none of these states apply.

Unlightable Candelabrum (3.1.0 to 3.4.3)

When the Candelabrum of Invocation is lit while not on the vibrating square, it loses half of its remaining turns of light. If this occurs while it has exactly one turn left, the turns remaining are rounded down to zero, but the attached candles are not removed as would occur if it burned down naturally, so no more candles can be added.

Destroyed Book (3.1.0 to 3.4.3)

It is possible to destroy the Book of the Dead by walking into lava without fire resistance. Doing so will destroy every fire-vulnerable item in your inventory, and the Book of the Dead is not exempt, so a hero that survives via an amulet of life saving will find the Book lost forever. This was fixed in NetHack 3.6.0, where the Book is protected against fire of any kind.

Missing Vlad (3.6.0 to 3.6.7)

If Vlad the Impaler has the Candelabrum stolen from him, he'll be able to shapeshift as other vampires do. If the hero dies and leaves bones with a shapeshifted Vlad in the level, he'll be missed by the code that would strip him out of the bones file. When another player loads those bones, Vlad will be present, and as a unique monster, he won't be generated again. This leaves no way to get the Candelabrum in that game.

Astral hangup (3.0.0 to 3.6.7)

On the Astral Plane, after offering the Amulet of Yendor to your god but before ascending, if you save the game via hangup, when you load, the Amulet will be gone, making ascension impossible.

Variants

SLASH'EM

In SLASH'EM, the six innermost doors on the highest level of Vlad's Tower are indestructible, protecting Vlad the Impaler and the Candelabrum of Invocation. They are opened by the alignment keys, artifact skeleton keys which can be destroyed like any other item (e.g. eaten by a metallivore). While the destruction of the keys would seem to render the game unwinnable, it does not: Vlad can be woken by such means as a drum of earthquake or a psychic blast from a tame mind flayer or a hero polymorphed into one, and he will then warp to the player, carrying the Candelabrum. Alternately, an appropriate covetous monster can be lured to his level, which will then warp to him and wake him up.

A considerably tougher situation can be created by trapping oneself inside Vlad's chambers, with the keys either destroyed or simply outside. This is actually not too difficult to do: quaffing a cursed potion of gain level on the floor below can leave you inside the locked doors, as can more obscure means such as re-locking the doors and eating the keys. As the walls and floor of Vlad's Tower are undiggable and the level is no-teleport, the only means of escape is some form of level teleportation or branchporting. A hero lacking such a means would be required to wait around until an appropriate item is generated from a death drop, and this is not guaranteed to occur before all suitable monsters are rendered extinct—particularly in SLASH'EM where non-humanoid monsters never leave death drops. In order to guarantee an eventual escape, a hero would need to exploit a monster capable of casting summon nasties, which does not respect extinction.

Sporkhack

If the quest leader dies, the stairs are automatically unlocked, to avoid issues with unwinnable games.

dNetHack

One of the artifacts, The Silver Key, can be used to perform the Invocation instead of the Bell of Opening. This lets the game become winnable even if blocked from the quest.