# Impossible
For situations where the game cannot be won, see Unwinnable.
NetHack generates an impossible message when something such as a monster or object reaches a state that it should not be possible to reach; it displays a short description of the problem, followed by "Program in disorder!  (Saving and reloading may fix this problem.)"[1] These messages are generated by the function impossible in pline.c.
Some bugs generate impossible messages.  For example, in NetHack 3.4.3, bug C343-33, "Sometimes a monster may remain hidden under a corpse that has rotted away.", can result in a player polymorphed into a hiding monster hiding under a non-existent object, generating the message "hiding under nothing?" when a monster tries to attack the polymorphed player.
If an error results in the session being unable to continue, the game will normally panic instead of issuing an impossible message.

References

↑ src/pline.c in NetHack 3.6.1, line 458

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