The O2 sensors, typically on the exhaust, one for each bank of cylinders, evidently sometimes also around the catalytic converters (?). Misfiring will usually create an O2 rich exhaust as it wasn't consumed by a proper burn, so the sensor will usually lean the engine out. Now you can start with some interesting cascading failures, as the leaner engine isn't running right but there is still one or more cylinders with a misfire so the exhaust is getting pretty dirty. It can start clogging up the O2 sensors, where they start by responding slower, and the dirty exhaust is also getting sucked back into the intake thru the EGR. Now you're set to start clogging the EGR valve and an EGR feedback sensor if you have one, and maybe an Air Control Temperature sensor if it's installed downstream of the EGR port in the throttle body. Now you also start getting the throttle body and plate dirty, or maybe just dirty more quickly, all of which makes for rougher running, a dirtier exhaust, slower or misfuntioning O2 sensors, etc., etc., and eventually maybe the converter starts getting clogged, the rings start gunking up, you start burning more oil, which really makes for a dirtier exhaust. Now you have an exhaust consisting of partially burned oil and fuel mixed with soot, coating the engine intake thru exhaust, and it's also blowing by the rings and getting into the crankcase. The much dirtier oil is getting sucked into, you guessed it, the intake thru the PCV valve, and it's competing with the dirty exhaust on which can clog the engine up the quickest.
The lesson is change the plugs once in awhile, even if they're hextuple plutonium plugs advertised to last for the next two cars that you'll own, as they really won't last that long :^) Plugs are cheap, as plug wires, relatively speaking. Change your O2 sensors once in a great while too.
It's the automotive universe version of the butterfly effect, but I guess we should call it the 'spark plug effect' :^)