I know it's my own damn fault, and I just need to be careful EVERY TIME I start a trainer battle but ...
... seriously.
I would never have got it right if it wasn't for Wowhead, and even then, I have been failing at the checklist over and over again:
* Each battle, get the pets sorted out and heal them
* Put the falcosaur first
* Now, summon the falcosaur
* Now, and only now, actually start the battle and win without killing the falcosaur
What gets me is fleeing the fight (because falcosaur dead or wrong team or whatever), then starting again ... but the falcosaur isn't out. Because, why would the pet you had out, that you explicitly summoned, still be out? That would totally make no sense, right?
Yes it's my fault but still.