Have you ever seen it?
 

The elf with real BIG muscles that tightens up transmission fill plugs when you are not watching!

I changed the oil in my 915 transmission yesterday. I changed from Swepco to Kendall.

I was the guy that put in the fill plug on the side of the transmission two years ago. I figure I was real strong back then or some elf snuck into my garage and used a impact wrench to tighten up the fill plug. Fortunately my plug is in good shape, and I hosed out the gunk in the plug with brake cleaner before I tried to remove it. I had to use a two foot cheater bar on the end of the wrench and push HARD and it came loose with a POP. I know I did not tighten it up that much. I sure did not torque it down real tight this time. I can't imaging how it got so tight. And yes, I was smart enough to take out the fill plug first.

Corrosion?

I doubt that. The threads were clean. Once it broke loose it was easy to unscrew.

Did you use any anti seize?

Next time get the trany good and hot before you try to loosen the plug. I had one I couldn't budge cold, but after some spirited driving, out it came. Let it cool down before you put the plug back in.

I believe heating/cooling cycles would be the possible "elf".