A course can be well organized around the wrong idea of what beginners already know.

Structure is not the same as accessibility. A course can be beautifully sequenced and still begin one step above where its learners actually stand, because the person who built it forgot what it was like not to know the basics. Everything after that first missing step feels like personal failure to the learner, when in fact the gap was in the design.

Whenever you explain something regularly, find out what people know before you start rather than assuming. Ask one new person to tell you where they first felt lost. That answer is usually a single unstated assumption, and fixing it helps everyone behind them.