AFAIK vehicle manuals say owner must replace spark plugs every N miles. Suppose he fails to do so and a single spark plug unexpectedly dies in an engine with multiple cylinders right in a freeway. What likely happens to the vehicle?
If you keep driving it that way for very long, the fuel that's pumping through the non-firing cylinder will contaminate the catalytic converter. That can result in the catalyst overheating and melting, possibly blocking the exhaust in the process (BTDT and cats are not cheap...). If you're really having a bad day, the cat could theoretically catch on fire and possibly result in a full-blown vehicle fire.
Spark plugs are normally a wear and tear kind of part instead of an "all or nothing" kind of failure. Over time, they get worn or build up deposits from years of use and they become less effective and efficient. I have rarely ever heard of a spark plug failing 100% all of a sudden. Over time, it would contribute to reduced gas mileage, rough idle, and sluggish performance. They recommend them to be replaced at a certain interval to keep top running performance. It is a very common part of a "tune up".
If it did some how fail, I would imagine it would be like the time I had a car where a spark plug wire got hit by the cooling fan and cut it. The car lost a lot of power as it was only now running on 5 out of the 6 cylinders. The car had only a top speed of like 45/50mph and it sounded like it was working really hard and very sluggish, so I pulled over immediately to investigate. It was very noticeable, but it did not present any immediate hazard other then requiring me to slow down and get off to the side of the road.
It should carry on running, albeit somewhat lumpy. I've seen a car (a 6-cylinder BMW) spit out a plug on the motorway, it carried on running quite happily, just got noiser and lost some power (as you would expect!). The owner pulled over at the next exit, found out what had happened and drove to the nearest parts shop to get a new plug.
No one has mentioned vibration yet.
When one of the spark plug wires went 'bad' on my Chevy LS1, the spark plug failing to fire immediately translated to regular vibrations that changed frequency with RPM and became especially bad at idle.
Replacing the wires (and spark plugs) eliminated the vibrations completely.
One thing not mentioned here is that the unburnt fuel from a cylinder misfiring will find its way into the oil and cause symptoms similar to a flooded engine. I called a mechanic to try boosting the engine on a relative's car today, and he said it was extremely flooded, the spark plugs needed to be replaced and the oil changed, and he wouldn't be able to do it until next week.
When I looked at the engine to see if changing the spark plugs was something I wanted to do myself, I noticed two of the six were completely disconnected. The oil smelled like gas and the oil level was quite a bit over full.
Spark plugs can fail in a number of ways - most commonly, the electrodes get encrusted with carbon deposits, but the electrodes can be worn away, the ceramic insulator can crack, the threads can fail, or the body can crack apart. Generally the plugs degrade over time, causing weak spark, poor combustion and reduced gas mileage.
In most cases, a failure would cause a single cylinder to stop firing, causing strange engine sound and loss of power. An unlikely worst case failure would see part of the spark plug break off and bounce around in the combustion chamber, damaging the cylinder, cylinder head and piston until it was crushed small enough to be spat out the exhaust port.
On most vehicles spark plug replacement is cheap and easy, and is worthwhile maintenance
I've completed a 2 hour endurance race on a vintage motorcycle running on only three of four cylinders due to an ignition fault. While I knew something wasn't right - the inability for my 1000cc four-cylinder to easily pass a 500cc single on straight-line acceleration was a big clue! - the engine ran smoothly enough to continue without danger.
I had a plug go out on my 1991 Honda Accord. On my way to work the other night my car was running fine when all of a sudden it sounded like a piston smacking a valve. Obviously, it lost a lot of power. When I took a closer look, I observed a plug wire popped out of the hole, so I put it back in the hole, thinking maybe that’s all that happened. But it blew off again as soon as I started it.
Closer examination led me to discover that the plug actually busted inside itself (which was new to me). The smacking noise was the inside of the spark plug being pounded against the outside of the plug from the combustion of the engine. So the whole inside of the plug was moving up and down while the outside of the plug was stationary. I put a new plug in, and I’m back in business.
The engine starts to misfire (for obvious reasons). Unburnt fuel gets passed through the exhaust manifold and you'll start to smell it. In the worst case, you might have a fire in a part of the car where you don't want it (which is a soft way of saying you might burn to death).
But I've driven 20 miles + with a misfiring engine and nothing serious happened except for poor consumption figures.