What is a common mistake when writing a reflective essay?

A mistake in a reflective essay is to write a reaction instead of a reflection.

As a short example (of just two sentences) about "How have I developed as a high school student?"

Reaction: My high school is very large and diverse. The class sizes are too big and detract from my ability to learn.

Reflective: Attending a large, diverse high school allowed me to develop more mature thinking about multiple cultures and the complexity of the world. Within large classes, as a microcosm of real life communities, I learned to test out my assumptions and to move into more enriched understandings of others.