Stock without a recipe
I stopped measuring anything for stock about a year ago and it has been better since, which I suspect says more about the recipes than about me.
The working rule: bones and trimmings cover the bottom of the pot, cold water two fingers above, and then nothing happens quickly for four hours. No boil. A boil emulsifies the fat and you get something cloudy that tastes flat no matter what you do afterwards.
Aromatics go in for the last hour only. Carrot and onion cooked for four hours give up everything they have and then start giving up things you did not want. Celery is the worst offender here.
Salt at the end, or not at all if it is going to be reduced later. I have ruined more stock by salting early and reducing than by any other single mistake.