Accidentally put 12V on 6V battery

I assume you mean 'jump start a car with a 6V electrical system'. If the problem was really dead battery, and the 12V jump got the car started in about 2 seconds, you're golden. If you hook a 12v charger to a 6v battery it will charge like heck for a few minutes then get real hot and start boiling off hydrogen. Dont come around with a cigarette.

WTF do you mean jump start a battery? A battery doesn't need a jump start.