Tragedy of the Blazers' Edge commons:

Consider a pasture used for grazing cows which is used collectively by many herders.  Each herder is trying to make as much money from his cows as possible which means maximizing their yield (fatter cows or more cows).  There are a positive and negative aspect to consider:

Positive: The herder gains profit for each pound

Negative:  Each cow degrades the pasture slightly

The distribution of the positive and negative results is unequal.  The individual herder gains all the profit from selling more or fatter cows, but the cost of (over) using the pasture is shared by the group.  A rational herder will see the benefit from grazing more cows outweighs the (shared) cost of the degradation of the pasture.  All of the herders will reach the same conclusion and put more cows on the field leading to overgrazing and a potential collapse of the pasture.  The individual gain outweighs the individual share of the total cost eventually leading to a disaster.

Now consider the Blazers' Edge sidebar used by Blazer fans for expressing their opinions about their favorite team.  Each poster is trying to have his/her opinion heard, appreciated, "rec'd" and commented on by as many fellow fans as possible.  There are a positive and negative aspect to consider.

Positive: The poster gains recognition/appreciation for his/her opinions for every post made.

Negative: The sidebar gets more crowded with every post made.

The distribution of positive and negative results is unequal.  The individual poster gains all the recognition/appreciation from his/her post, but the cost of (over) using the sidebar is shared by all of Blazers' Edge.  A rational poster (perish the thought) will see the benefit from posting more outweighs the (shared) cost of a clogged sidebar.  All of the posters will reach the same conclusion and post more leading to a highly congested sidebar where fanposts are not visible long enough to get many comments/recs thus negating the whole reason for posting.  The individual gain outweighs the individual share of the total cost eventually leading to a disaster.