Initially choosing a Secondary for damage rather than support- I ended up using almost all the Crowns from an Epic Bundle to buy back my TP's so I could get support cards like Feint, Satyr, Tower Shield, etc. instead of Helephant and Wraith.
I regret learning a third school before moon. I eventually had to buy my training points back again. That's right, not that first time, only now it was crazy overpriced. Also, I regret accidently selling the epic bundle robe to a regular merchant.