Common mistakes when applying opacity to 3D models include ignoring render order (incorrect transparent layering), overusing partial opacity (hurting performance or clarity), and neglecting material-lighting compatibility (unrealistic results).
Render order issues occur when transparent faces render behind opaque ones, breaking depth and light consistency. Overusing partial opacity (e.g., 50% on all components) can make models look muddy or slow down renders. Material-lighting mistakes happen when you make an opaque material (like plastic) transparent without adjusting light interaction, leading to unnatural glows or shadows.
To fix these, enable alpha sorting in your 3D software (for proper layering) and test opacity on one material at a time—focus on key details (glass, thin fabric) instead of applying it everywhere.