What is Rule 0.5?
1.
A rule of table top gaming. If the DM/GM is being an Ass/dumbass/idiot and/or abusing rule 0, the players can choose to override him by leaving his game and starting their own.
An alternate version involves Smacking the DM over the head with the Dungeon Master's Guide.
DM: The ghoul rolled a 28 On his check to resist your Trip attempt.
Player: How the Hell does a ghoul get a 28?
Dm: With His +8 grapple modifier.
Player: But it is a Strength check, not a grapple!!
DM: I am the DM, Under Rule 0 I say He uses his Grapple Modifier.
This is where Rule 0.5 Is invoked
Every player at table: Smacks DM upside head with books and Leaves game.
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