An abrupt and perhaps unfair way of ending a game, in response to a player trying to do something that appears completely harmless. If the PC is on the roof and the player types "jump", any resulting death message would not be a surprise -- the player has asked for it. But consider the following:
You are in a sunny meadow. A cheerful brook lies to the north.
A flaming meteor screams down out of the sky and smashes you to pieces!
***** You have died. *****
If, for example, the player had been warned in advance that a meteor deflection screen protecting the meadow has been disabled, then the above sequence would not be a complete surprise, and the sudden death would not appear quite so unfair.
In general, if the PC must die in order for the player to advance in the plot, then most IF players would probably consider the game to be unfair. If the room description provides even a subtle clue that a particular action might be harmful (for instance, if the "cheerful brook" was instead described as an "ominous effluvience"), then the player at least has the chance to invoke ">save" first.
Recent experimental IF uses sudden death in creative ways. Both Aisle (Sam Barlow; 1999) and Rematch (Andrew D. Pontious; 2000) are single-move games that must be replayed multiple times in order for the player to uncover the plot.