Me, I learned that if you want to create a cave, you can just pick a stone model, size it up, and put the player inside.

I learned about baking light maps

That Android can't open local files with default permissions

The difference between Forward / Mobile / Compatibility Renderers 

and how to make a VR Game 😁

I learned the audio bus system, recording, playback, etc.

I also learned that next time I should test it on a fresh device without an existing shader cache...

I learned about physics process priority. Unfortunately, I learned after the deadline and the bug (hands lagging behind) is in the submitted version. :(