I own a 2000 Jaguar S-type 3.0 and it won't shut off. I turn on the car and when I turn the key to shut it off it will stay running, I can even take the key out of the ignition and drive the car. Does anyone know what is wrong with it?

There is a phenomenon is called ''Dieseling'' and can have
several causes.

Basically there is some alternate source of ignition apart
from the spark.

This could be glowing carbon deposits, a sharp thin edge of
metal glowing, using a poor(low octane) fuel. too much
methanol in the fuel, wrong heat range spark plugs and
several others.

This used to be quite common with carburated motors however
with fuel injection and ignition off there should have been
no fuel entering the cylinder so it should not have been
able to not run on.

I would conclude that although the key was removed the the
ignition and fuel injection systems were still energized.

Things to check or have fixed.
1) timing
2) motor running (Idling) to fast
3) the motor may be ruining rich (stuck choke )
4) over heating
all of the above can cause run on..It might need a tune up
if it happens again here's a couple of things you can do to kill (stop) the motor.
1) if its an automatic leave it in gear when you shut it off the extra pull on it well stop the motor.
2) if its a manual transmission put it in gear and let the clutch out with your foot on the brake and it well stop the motor..

get it to a repair shop asap..

good luck