An example can become so memorable that students mistake it for the entire principle.

A vivid example can eat the principle. Students remember the story, the striking case, the memorable illustration, and they carry it forward as though it were the rule, applying it where it does not fit and missing the rule where the example does not resemble it. The teaching device becomes the content.

Give two examples that differ, so the shared structure has to be extracted. When you are learning, try to state the principle without the example attached. If you cannot, you have the illustration rather than the idea.