English: Nondisjunction of sister chromatids during mitosis:

Left: Metaphase of mitosis. Chromosome line up in the middle plane, the mitotic spindle forms and the kinetochores of sister chromatids attach to the microtubules. Right: Anaphase of mitosis, where sister chromatids separate and the microtubules pull them in opposite directions.

The chromosome shown in red fails to separate properly, its sister chromatids stick together and get pulled to the same side, resulting in mitotic nondisjunction of this chromosome.