Would you be unnerved if you cracked an egg and found no yolk inside? Earlier this month a Japanese woman boiled some eggs and when she sliced one open, she found only the white inside. No yolk to be seen—at least on the photo she posted on Twitter.

Sometimes hard-boiled yolks hide themselves by drifting far to one side of the egg, but in this case it seems that the yolk is actually missing. What’s going on here?

Yolkless eggs are actually common enough that chicken keepers have a number of names for them—fairy egg, witch egg, rooster egg, oops eggs, dwarf egg, wind egg, and, most commonly, fart egg. This last and most evocative name derives from the theory that the eggs are produced by chicken farts. In fact, they are formed when a piece of reproductive tissue ends up in an oviduct, which grinds into gear and makes an egg around errant tissue.

Often, though, fart eggs are significantly smaller than normal eggs; perhaps what’s most impressive about this yolkless egg is how large it is. Usually these eggs are identified and kept out of supermarket-bound packages, and this one just happened to slip through. (This does seem like a missed market opportunity—a dozen egg-white only eggs would be perfect for people who choose egg-white omelets or recipes that call only for egg whites.)

Really, what’s most amazing about the surprise of the yolkless egg is that anyone’s surprised at all. The sorting and packaging of eggs obscures all the different ways egg production can go wrong—like many natural processes, egg-laying can get messy.