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.
There are, however, a few mostly quest-related ways that a game may be rendered unwinnable, mostly by accident.
- Before getting permission from the quest leader, converting your alignment at an altar,
- Before permission, quest leader dies or levelports down,
- Before permission, getting expelled by angering the quest leader or failing the alignment test seven times (exception: neutral non-wizard),
- Before proceeding down, converting your alignment at an altar (exception: was lawful or chaotic, and quest portal still present)
- The Candelabrum will become permanently unusable if you turn it on while it has fuel for exactly one more turn and you are not on the vibrating square. This is Bug C343-424.
- Destroy the Book of the Dead before the invocation by falling into lava without fire resistance but wearing an amulet of life saving.
- With the Amulet of Yendor remaining in the quest dungeon branch, getting expelled (exception: leader still alive; must be a wizard, neutral, or able to convert to neutral)
- On the Astral Plane, after offering the Amulet of Yendor to your god but before ascending, saving the game via hangup
These actions make it impossible to obtain the Bell of Opening from the quest nemesis, or to get the Amulet of Yendor back out of the quest branch, or to perform the Invocation. All of these are necessary to win.
Converting your alignment
If the player unsuccessfully performs an altar conversion by sacrificing at a cross-aligned altar with negative alignment record, he or she is converted to the alignment of the altar. That means permission to proceed down cannot be obtained from the quest leader at all. A player's alignment cannot be converted back to any previously held alignment by means of an altar, and the quest leader will "see through" a helm of opposite alignment.
You cannot go down or level teleport from the quest home level even with prior permission if the player is not of his or her original alignment. This can be overcome with a helm of opposite alignment.
Note that simply putting on a helm of opposite alignment does not render the game unwinnable -- removing the helm (after uncursing, if necessary) restores the character's previous alignment. You cannot put on the helm in the quest branch. The message you get if you try is somewhat misleading.
A second case is if the quest portal has been removed because you have angered the quest leader, or have failed his alignment record test seven times. Don't throw the silver bell or the Amulet of Yendor at him!
If you threw the Amulet of Yendor at the quest leader, it is trapped in the quest branch. Once the portal is gone, the only way to get it out is to get expelled again while carrying it. The quest leader still has to be alive for this to work.
Entering the quest at all requires the player is neutral, is not a wizard, and has previously entered the quest branch: the player can wish for the Eye of the Aethiopica to branchport to the quest. (You have to have visited the quest once already because the Eye will not open a portal to a branch you haven't been to, and it will only drop you as deep as you have already gone before.)
Dead quest leader
If the quest leader is killed or level teleports before you get permission to go down, the game will also be unwinnable. (Monsters will read scrolls of teleportation when they are low on hit points.) Luckily, the leader is generated meditating, and a peaceful one will only be attacked by pets under conflict, so a levelporting leader is extremely rare. Due to a bug, a petrified quest leader brought back with stone to flesh or one turned undead will not grant permission. Doppelgangers can imitate your or other quest leaders, but they will not grant permission, either. (Wish for and stone-to-flesh a statue.)
The same restrictions apply to expulsion from the quest, in case you are trying to leave it with the Amulet.
SLASH'EM does not actually add any new ways to make the game unwinnable, although it certainly adds ways to make it nearly so. For example, although one needs two of the three alignment keys to reach Vlad the Impaler, lacking them, it is possible to wake him up with a mind flayer's psychic blast. He will then teleport to meet the player, and be carrying with him the Candelabrum. Also, locking vital items behind the doors and destroying the keys doesn't make the game unwinnable either. One can retrieve the Amulet or any Invocation item by luring an appropriate covetous monster into the third level of Vlad's Tower, and the monster will then retrieve the items for the player.
If the quest leader dies, the stairs are automatically unlocked, to avoid issues with unwinnable games.