That's the insidious part of coil failure, at least in my experience, there is no warning whatsoever. My experiences were these:
first failure- drove to work as normal , car seemed perfectly fine. Came out at end of day and started up car. Car sounds a bit funny and has a noticeable vibration- running on 5 cyl, Damn, coil failure.
Failure #2 sitting at stoplight car IS running fine, all-of-a-sudden its running rough and sputtering, bad vibration, Damn, coil failure.
(at this point the light-bulb should have gone off in my head , but didn't)
Failure #3 drove home from work, car ran fine, came out next morning and sputter, vibrate, Damn another coil failure.
Light-bulb in head is now spotlight in face, Doh! All coils get replaced :D
I was lucky for several reasons, I live less than 5 mi. form my work , I wasn't far form home when the coils failed and I have teenage drivers (and extra cars) so I'm never stuck without a vehicle or at least a ride. It certainly could have been much, much worse.