Tragedy of the commons
The Tragedy of the Commons was an article published by Garrett Hardin in the journal Science in 1968.  It describes a problem where many people with their own ideas can make something they all share worse, even if no one wants to. For example, even if no one wants to pollute water because that makes it unhealthy, it can still end up like that because so many want to use the water for their own reasons, like washing and throwing away rubbish. Each person thinks that his small bit of pollution of the water is too small to affect the quality of the water, but because there are many people the total effect ends up making the water too polluted for mostly anybody to use for drinking or even washing. This mostly occurs in slums and other overcrowded places like refugee camps.