Auto Transmission

Overpressure eventually causing seal failures most likely. Leak-by and slipping is likely result.

Undoubtedly you'll have a hard time collecting but it would appear a dealer-aided if not totally caused early failure is indicated.

overfilling will not affect pressures. What WILL happen in all ACTUALITY, is that the fluid will foam from being in contact with the rotating mass of parts. THEN, the result of the foaming is slippage due to the LACK of proper pressure on the clutch packs and band(s). Depending on the transmission and the length of time it was allowed to be this way will determine the extent of the damage. Simple solution? Yes,, let some fluid out. DUH.

But considering the source of the OP, i'd say it's probably toast.

When it heats up and expands it will have to go somewhere. Depends on how much reserve space is in the oil pan. If there isn't enough room it could blow out the dip stick tube or blow out a seal or gasket. It's best to follow directions.