Common mistakes when flat shading polygon 3D models include incorrect face normals, inconsistent polygon winding, and mismatched lighting setups.
Incorrect normals make faces look unlit or inverted—flat shading relies on each face’s normal to calculate light. Inconsistent winding (clockwise vs. counterclockwise vertex order) can cause faces to render incorrectly or disappear. Mismatched lighting, like soft global lights, washes out the sharp, blocky look flat shading is meant for.
For beginners, start by checking normal directions in your software (e.g., Blender’s “Show Normals” tool)—it’s a simple fix for most early lighting issues before adjusting lights.