If players find an exploit there is usually someone willing to use it. Even if it ruins the fun of others around them also playing the game.
Pokémon Go has been very successful there's no doubt, and one of the big features for players is locating and fighting for the possession of the Gym locations dotted around their area, hoping to turn the area all red or blue or yellow for their chosen Team. However many people are finding that Gyms can no longer be won because the person in control... has placed an Egg inside the Gym.
When you win the possession of a Gym you must leave behind Pokemon there for passing challengers to do battle with, and your fellow teammates can add their own to help boost the Gym's defences against being taken away from your Team's possession. However, an Egg is an unhatched Pokemon. They therefore have no character data or stats assigned to them and therefore cannot be battled with - meaning that the Gym can't be switched from a Team's allegiance and neither can anyone gain XP from the Gym via battling. It also allows the player who placed the Egg in there to continue to earn the Pokécoin rewards they get for holding onto the Gym without any fear that they'll lose it.
So far numerous Gyms in the New York area have been locked up like this, and as news spreads Gyms across the world might soon find themselves forever frozen in a Team's possession.
At least until Niantic are able to patch this exploit out. God knows what'll happen to all those Eggs then... maybe redistributed to more responsible Trainers?