Question on Automatic Transmission Fluid Change

I have a 2008 Clubman S, and yesterday I tried to teach my "close to licensed" son how to do vehicle maintenance as I do all my own. Although we were intending to do an oil change, he incorrectly pulled the drain plug from the transmission. He walked the plug over to me and showed it to me, which is when I realized it was the wrong plug and quickly put it back. When I crawled under the car, only a small amount was coming out. The plug was out for probably around 30 seconds, and I did not see how much came out initially. It was draining into a oil catch that already had some old oil, so there was no way to determine how much drained. I'm concerned not knowing how much came out.
Here are the facts: I have a slanted driveway, so with the car backed in, even though the front of the car was up on ramps, the car was basically level. The overflow tube was NOT removed, so anything that spilled out was over flow. I don't know the temp of the fluid at the time it happened. The car had been sitting approximately an hour since driving before we started. I know there's always a chance that the factory overfilled it in the first place which is why some was coming out, but should I be worried? Thanks for any input.

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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: MINI Cooper - Automatic Transmission Fluid Change

[Nick at Pelican Parts]

Unlikely it was over-filled at the factory.

Use the filling and level checking procedure to see how much you need to add. Don't discount the small amount drained. Trans damage can occur if the fluid is low.

- Nick

[Dave at Pelican Parts]

Could it have been the fill plug instead of the drain? If so, then the small amount coming out was due to the car not being completely level.

Regardless, Nick is quite right about making sure it's actually full. I have personally killed two transmissions (in a Honda) through chronically underfilling them. Due to checking them when the car was not level...

--DD