When you overfill an automatic transmission with too much transmission fluid you will cause damage to the automatic transmission which will overtime cause problems.
Overtime the transmission will have problems and then it will eventually go bad on you leaving you stranded.
Adding too much transmission fluid or overfilling your transmission can cause foaming and transmission fluid starvation for the transmission which is very damaging because when the transmission doesn't get the proper amount of transmission fluid the gears and bands are not lubricated properly.
This can cause the transmission to completely fail so if you do overfill the transmission fluid you need to have the excess removed by dropping the pan or taking the vehicle to a shop and have them suck the excess transmission fluid out of your transmission.
It's best to add just a little bit of transmission fluid at a time until it's full and check it several times while adding a little bit of transmission fluid to avoid overfilling the transmission.