Burnt roux! Darn! It's easy to do. My best advice is to ignore any advice that says to cook roux over high heat. Make sure the fat you're using is NOT heated to it's smoke point Butter burns over high heat, so cook it over low. Bacon fat is VERY hot when you pull bacon out of the pan, so set the fat aside to cool slightly before adding flour. Always cook roux over medium low heat and take your time, slowly increasing heat as your confidence increases. Not stirring enough is another culprit. In the beginning, do not walk away from it, stir constantly. As it cooks, especially if you're cooking over medium-low heat, you can pause for a 10-20 seconds, but do not walk away.