A D20 Modern or Dungeons and Dragons 3.0/3.5 rule variant.
Anytime a ranged flame attack is used against a target there is a possibility that they will become engulfed in flames. If a target gets a saving throw and they make it nothing happens, if they fail a d20 is rolled. If a natural 20 is rolled the target is engulfed in flames. Being engulfed means that: the target immediately takes full damage for the attack (no damage roll needed), the target must take two full rounds to be able to put himself out with each round taking damage from the flames (this damage being 2d6 instead of 1d6 being on fire). If the character tries to do anything else besides put himself out he takes a -4 penalty to all actions, attacks, skill checks, and saves. A large body of water will put he character out immediately and it will take 12 gallons doused over a creature to quench the flames.
Creatures who cannot catch fire, made of metal, means of magic, certain elementals, or etc, are immune to this effect. The DM should use his best judgement when determining if engulfment is appropriate.
