Flame guy is a character/bug in the game Blood. When an enemy dies from fire, usually from a weapon like the Napalm Launcher or Aerosol Can, he will burst into flames and run around, causing slight fire damage to whatever it touchs, until he collapses into a smoldering pile of flesh. The "Flame Guy" bug happens when the enemy continues to run around infinitely. Usually this happens when the enemy runs into a wall and becomes clipped (trapped between separate areas).
Flame Guy cannot harm the player after his "living time" has expired, but is invulnerable to hitscan weapons like firearms or the Voodoo Doll. Usually an explosion will deliver the killing blow, as it gibs the enemy. Flame Guy is also resistant to water.