Mixed ability class. I made a call about who should continue to study Pythagoras, and who should move on to learning about Trigonometry.
In the unit assessment, one glorious student (who I had decided should stay on Pythagoras), flew through the Trig questions, doing better than a lot of the students who had learnt it in class. He had gone home and asked his cousin/youtube to teach him.
Outstanding example of the dangers of overly rigid differentiation. What should I have done differently?