This one has caused nearly every driver to have a heart attack, followed by relief, followed by irritation that lasts for days. The gas cap.
The check engine light comes on while you're driving. Your heart stops. Should you pull over right now? Is a piston about to come flying through the hood? What does your automotive robot overlord want you to do? How do you appease him?
You drive straight to the dealership in a panic. The mechanic takes out his diagnostic device, plugs it into the car's computer and then he gives you the bad news: the gas cap isn't screwed on all the way. You nearly faint with relief that it isn't a $1,000 problem. You wouldn't have lost your life had you continued on your way.
Wait. You wouldn't have lost your life. This was not an emergency. It didn't have anything to do with the engine, which is important. It had to do with the gas cap, which is important, but hardly in the same league.
If you're a BMW driver, your car will give you a bit of a reprieve. If the service engine light comes on and you think, "Huh, I did just get gas. Maybe I'll check that before I continue with my panic attack," and screw the cap on yourself, the light will go off without the assistance of a diagnostic tool to clear the code. Crisis averted.