Pokémon Go players have found a way to permanently win a gym for themselves - by placing an egg in the location instead of a Pokémon.
Numerous gyms in the New York area have been claimed by people using the exploit, Reddit users report.
A gym with an egg placed in it instead of a Pokémon cannot be fought or won back.
Eggs are used for hatching Pokémon and therefore have no character data or stats. It is only upon being hatched that they actually reveal the Pokémon inside.
The exploit is an annoyance for anyone playing nearby. It means you can't change the gym's allegiance or gain XP via battling there.
But it's also a huge exploit for the sneaky Pokéfans who put the eggs there in the first place. Players earn themselves Pokécoins - the app's in-game currency which you can also buy with real-world money - by holding gyms. Anyone using the exploit can now claim a Pokécoin reward for every gym they find.
Without any way of dislodging their Pokémon, it looks like other players will just have to wait for developer Niantic to patch the exploit out at some point in the future.
It's not the first time there's been a mystery around the Pokémon within gyms. Last month saw the saga of how an Ohio woman found herself in the headlines worldwide for having the legendary Pokémon Articuno. After a bruising few days during which the internet branded her a liar, developer Niantic finally confessed it had randomly sent her the Pokémon after all.