An in game year ago, I made pits throughout my world to capture animals. I caught one rooster from here and used it in my farm. I recently killed all the roosters by accident (they run too fast for me to identify them as I swing my falx...) so I went to capture this guy. He's hard to catch, he has usually just taken the bait out of the trap and escaped. I come back the next day to try again. The most recent time I came to get him, he had laid an egg? in the pit. There are no hens nearby, this is on a thin shoreline bordering a huge mountain, so not really anywhere they can hide. Even if there was hens, how would they have got in and escaped, or have been well fed enough to lay an egg? Very strange mystery. These chunks are usually unloaded so I think the game just made a mistake when reloading them.