I know someone that is adament that their swordtails cross breed with there guppies. There evidence is solely that some guppy fry develop swordtails.
I can't convince them that it is not the case. I even said to them "If guppies and swordtails can cross-breed then humans and monkeys could too, it's the same thing. And if monkeys and humans could cross-breed then maybe some un-name famous person would have some questions to answer to.'