Lemons and grapefruit are hybrids
Really! Thanks to Maggie Koerth-Baker at Boing Boing, I now know that lemons and grapefruits are accidental hybrid fruits. Lemons come from a mashup of the citron and orange in India, and grapefruits come from an Asian pomelo and orange that hybridized in Barbados.
More on hybrid fruits in this BBC article.