fire out the exhaust, 4 causes im aware of:
1: Big ass fireball, or constant flames = flamethrower kit. the way i do these is a i weld a bung in the exhaust and use a blaster ss coil to fire that spark plug. at the same time I run to the same momentary switch a relay that cuts ignition. fllor it, it the button, houston, we have ignition
2. lil blips, purple flames, things like that. my crx and the impreza 2.5RS do it, its caused by a big cam, mixed with some hot exhaust. unburnt gas gets pushed out the exhaust. blip blip
3. decent lil blast of fire. overly rich conditions, caused by exactly what bobski said, or by bad fuel tuning, especially on cars like WRX and EVO;s. I know for a fact this can happen, mainly on non catted cars. our WRX almost does it, when i run the BOV full atmosphere.
4. coolest flames ever, antilag system, for laucnhing and FTS. basically this is ignition cut, instead of fuel cut like what i use for launching. it keeps the turbo at full boost. also createds lots of fire dumping unbrunt fuel out the back in an extremely hot condition. this kills your tuebo extra quickly.