Carb'ed cars don't have very good fuel management, and hence take a bit of unburned fuel through the cylinder and into the exhaust.
If you wind a turbo car up hard, you get traces of fuel into the exhaust when you shift gears (as the large turbo spools down it momentarily pulls some fuel through during the valve overlap). An FQ-400 is a good example.
A lot of turbo cars have blown their exhaust off from what is called detonation. It's similar to pinging, and can blow a cylinder block/head/piston, or the exhaust, depending on how far the unburned fuel gets. Overboosting a turbo can cause this easily (along with pinging).
Oh, and turning your ignition off and back on while driving is really, really bad!!!
When i spent time in Dubai, my mechanic told me of an arab who used to do that for fun, at 100mph, in his new ferrari (348 i believe). He turned the ignition off for a fraction too long, and blew the engine cover and exhaust clean off. He then gave the "wreck" to the mechanic (and probably bought a lambo), who after a couple of thousand pounds had a brand new ferrari!!