Depends on the car. With a Honda MT, you fill until the fluid (was motor oil at one time) comes spilling out of the horizontally mounted fill hole. I don't think anything bad could really happen if you manage to overfill that kind of tranny other than having to clean up the excess fluid. Even if you manage to fill it an close the bolt before it had a chance to drip out until it's perfectly level, that shouldn't do any damage.

Other transmissions might not be positively sealed like a two bolt MT. My WRX has a dipstick, where the dipstick tube also doubles as the fill hole. I could imagine filling that until it comes out the dipstick tube, but that would take a lot of fluid.