Answer:
People will overuse or misuse a common resource that is not excludable but is rivalrous.
Explanation:
The tragedy of the commons occurs when due to lack of regulation, either self-imposed, or imposed by a central authority, leads to the excessive use of a common good, that does not exclude users from its enjoyment, but that is rivalrous: the use of one user prevents the use of another user, and can lead to depletion.
A classical example of the tragedy of the commons is what happens with global maritime fish stocks. The global stock of fish is virtually non-excludable as long as a person or firm has the means necessary to exploit it: a ship, a net, workers, and so on.
Howerver, the global stock of fish can be depleted, as is the case in some areas of the world. This depletion prevents current and future users from catching and consuming fish.