Ok, that’s oversimplifying it. But imagine the surprise if you cracked open a chicken egg for dinner and found a dead gecko inside.
No no, this isn’t the holy grail that Creationists have been asking for as proof of evolution (which would actually disprove evolution if found). Nor is this the result of some mad scientist’s experiment. The truth is much more… well, not exactly mundane. The lizard crawled up the chicken’s cloaca and into the oviducts where it became an egg.
As Pharyngula puts it, “I don’t know about you, but a system that muddles excretion with reproduction and that allows random lizards to crawl up your butt and squat in your oviduct doesn’t sound like great engineering to me.”
Edit: deneb7 suggested that maybe the hen was mating, and when the cloaca inverted to suck up the sperm, it sucked up the gecko along with it. As for why an egg hen would be with a rooster, “They did it secretly when the farmer’s asleep :p”