In 1993, Wilma Stuart gave birth to two baby boys. They were fraternal twins, so some dissimilarity was to be expected. However, only one of the boys seemed to take after his parents of white Dutch heritage. The other sported a much darker complexion.
Wilma's pregnancy was due to in-vitro fertilization (IVF), in which her husband's sperm was combined with her ova in a petri dish. In an unforgivable breach of proper medical procedure, however, the pipette used to transfer material had apparently been reused after a visit from a previous sperm donor. Wilma Stuart's ova were fertilized by both men, and two of the re-implanted embryos matured into healthy young boys.
Strictly speaking the boys were only half-brothers, even though they were delivered as twins. They entered the medical literature as yet another documented case of heteropaternal superfecundation, a scientific term meaning "different fathers, multiple babies." Most such cases, however, are not the result of IVF, but rather more traditional conception methods.