Your neighbour Nigel is out of town on Halloween and is therefore unable to hand out candy to trick-or-treaters. Because Nigel still wants to participate in Halloween, he leaves a bowl of candy by his door and a sign saying "please help yourself". Within a few minutes, a few children came and took all the candy for themselves. This is an example of:

Tragedy of the Commons

Incomplete Markets

The Free Rider Problem

Missing Markets