In Dungeon! a wizard must choose to use a spell, let's say lightning, before the monster of a room is revealed. What happens if the monster is revealed and it appears to be immune to lightning, as shown by the "-" sign?
It is treated the same as if they rolled a number that was too low. They are defeated and suffer the usual consequences.
There are various different versions of the rules. The ones here show it on page 10, third paragraph.