Want to permanently capture a gym in Pokemon Go? Use an egg
A recently-discovered glitch in Pokemon Go allows trainers to permanently control captured gyms, by placing an egg to defend the location.
Once a gym is defeated and reverts to a neutral form, a player can claim the gym by inserting one of their own Pokemon. It turns out that if you manage to stick an unhatched egg (not an Exeggcute, but hopefully at this point you know the difference) in the open spot, you can block other players from attempting to defeat and claim it.
Unhatched eggs aren't treated as actual Pokemon in the game -- they have no battle model, animations, or stats, so a rival player activating the glitched gym will just see an empty platform and a CP 0 egg, which can't be fought. Because Pokemon gyms are controlled or lost based on their prestige, and prestige goes up or down based on battling, you guessed it: there's no way to lower the gym's prestige, and thus no way to defeat and take over the gym. The egg's there forever.
Annoying but harmless, right? Well, consider that controlling gyms for long lengths of time also nets you Stardust -- an all-purpose ingredient in powering up Pokemon -- and Pokecoins, the in-game currency which you otherwise have to pay real money in order to acquire. That turns this from a silly prank into a serious exploit (eggsploit?), and one we can expect developer Niantic to patch out ASAP.
I haven't been able to reproduce the glitch for myself (and neither should you, considering Niantic has started to crack down on cheaters with permanent bans), but you can see it in action from the video below.