Segfault

NetHack segfaults in certain rare situations where a function expects the game to be in a state that it is not, e.g. assuming that an object exists which has already been removed from memory. Depending on the operating system used, the behavior can be slightly different; on Windows, Nethack closes and often displays a message saying "NETHACK.EXE has stopped working"; on a public server the player will be sent back to the launch screen with no message at all; and usually the player's progress on the current dungeon level will be lost. The game in question needs to be recovered so the player can go on playing.

Less serious errors can cause a panic, which basically also means the game is in a state it shouldn't be in, but with the difference that the programmers thought about the possibility of it happening. Segfaults, on the other hand, are situations where the code doesn't lead to a panic in the case of a faulty game state.