I was helping out a friend yesterday who was facing some issues with a transmission-related CEL on his automatic 2003 Mazda6.
When we checked the ATF dipstick the transmission fluid was a clear light-pink color (so probably freshly-changed by a previous owner) and well above the max level mark.
The dipstick had two markings - 20 °C and 65 °C. It was well above the 65 °C mark when I checked it with the engine warm (say 60-80% higher). I also asked my friend to check the level later on with the engine cold. It was still well above the 65 °C mark.
In my experience, the old fluid is discharged through a drain plug, then new fluid pumped in through a fill plug hole. Once the new fluid starts dripping out of the fill plug hole, the amount of fluid is at the "right" level. This is also confirmed by this answer to a related question.
With this in mind, how is it possible that the transmission fluid was overfilled? Am I missing something? I suppose someone could have topped up through the dipstick channel itself but that doesn't seem like the right way to do this. Of course, I could be wrong about it.
My concern is that the dipstick might not reading the correct amount of transmission fluid due to some kind of internal blockage, especially since the ATF fluid looked pristine.