Overfilling the transmission with too much transmission fluid can blow out the transmission pump and seals and cause damage to the transmission.
Always constantly check the transmission fluid level with the engine running and the transmission in either park or neutral depending on what the transmission fluid dipstick states.
Add a little transmission fluid at a time and check the transmission fluid level after adding the transmission fluid.
If you overfill the transmission fluid you'll need to loosen the pan and drain the excess transmission fluid out or the transmission can suffer damage and need rebuilt.