Know Your Student
I was once running a train-the-trainer course and one of the trainees was having difficulty in choosing a suitable training project. As he was an engineer I suggested he teach the resistor colour code. (The resistor colour code is a numbering system where numbers are represented by coloured rings.)
When I was coaching him on 'Describe the Students' I emphasised how important it is to describe the physical characteristics of the students including colour blindness. I then heard laughter and chattering from the rest of the class. When I asked what was causing the laughter, I was told that the student himself was colour blind. I had made the incorrect assumption that engineers would not be colour blind. There's nothing like being hoisted by your own petard.