Ignition light

Most times the ignition warning light starts coming on it does it
stealthily - like a bit of a flicker, a glow or glimmer.  Maybe it
plays games after you have had some of the wiring circuits changed.
This time, suddenly, cruising along at 3500 rpm and it just came on;
bright and red, and stays on.
What would cause that?

Guy
(other than broken fan belt or angry Lucas LoD ?)