For anyone curious how it happens:

How does spontaneous sex reversal occur? Typically, a female chicken has only one functional ovary, the left one. The right ovary and oviduct are present in the embryonic stages of all birds but usually do not develop in chickens. Most cases of spontaneous sex reversal result from a disease condition that damages the left ovary. This condition could be an ovarian cyst or tumor or diseased adrenal glands, which cause the left ovary to regress. Residual tissue in the right ovary develops in the absence of the functional left ovary. This regenerated right gonad is known as an ovotestis and may contain some tissue characteristics of the ovary, the testes, or both. Steroidogenically functional, an ovotestis secretes androgen as well as estrogen. As a result, the hen develops male secondary sex characteristics. So, although the bird is genotypically female, it is phenotypically male. (Genotype refers to the actual traits coded for in paired genes; phenotype refers to the observable expression of those traits.) There are reports of hens with ovotestes producing semen, making them capable of fathering offspring. However, most chickens that have experienced sex reversal never lay eggs or sire offspring.

TL;DR if the left ovary stops functioning for whatever reason, the right one may develop into an ovary/testicle hybrid that secretes androgen which then causes male secondary sex characteristics to show up. In seemingly very rare cases they can produce sperm and thus offspring.