Physical Chemistry. First semester of first year of university. I couldn't understand anything that our 80+ year old professor was mumbling, and the slides he used were terrible (full of abréviations, diagrams missing steps, etc.) Although I was getting solid grades in all my other courses, I failed the first PhysChem midterm with something like 23%. I resigned myself to my fate and sheepishly told my mom (I was still living home at the time and parents were paying for my studies). She got a mad glint in her eyes, and asked for all the course materials. By next week, she had completely reworked the material and came up with new tables and diagrams to help explain the concepts. I was amazed at how simple it all really was. For example, atomic bonds aren't static but can "wiggle" around in several ways, and we can even calculate fairly easily the amount of energy required for each wiggle.