This problem usually occurs when a precipitate forms too fast. Foreign ions or molecules stick loosely to the surface of a growing crystal. If new crystal layers form quickly on top, those foreign particles get sealed inside before they can escape back into solution.
This is different from a solid solution, where an impurity ion actually swaps places with an ion that belongs in the crystal’s structure. In occlusion, the trapped material sits in small pockets or gaps within the crystal instead. This matters most in gravimetric analysis, a lab method where scientists weigh a precipitate to figure out how much of a substance was in a sample.
Trapped impurities add extra, unwanted mass to the precipitate, throwing off the final measurement. Occlusion is a common source of error in careful lab work, so chemists check for it whenever a measurement seems slightly too high. Chemists reduce occlusion by growing crystals slowly, often heating and stirring the mixture gently for a while after precipitation, a step called digestion.
Occlusion happens more often with fast-forming precipitates that have a large, loose surface area, such as gelatinous metal hydroxides, than with precipitates that form large, well-ordered crystals like barium sulfate. Analytical chemists routinely re-check a stubbornly high result for this kind of trapped-impurity error before trusting the final number.