In
NetHack,
a major disappointment. Though the
room you are standing in will suddenly be
lit, you
die the next turn from the
mumak you were hoping to
teleport away from.
If you were unlucky enough to have a cursed scroll of light, the room you are standing in will (naturally) be darkened instead of lit, adding insult to injury. Of course, this may actually be preferable, as other creatures in the room won't be able to witness your ignominious end and laugh about it with their friends later (and your pet can escape under cover of darkness).