Alex at Marginal Revolution has some interesting things to say on unintended consequences:
The law of unintended consequences is what happens when a simple
system tries to regulate a complex system. The political system is
simple, it operates with limited information (rational ignorance),
short time horizons, low feedback, and poor and misaligned incentives.
Society in contrast is a complex, evolving, high-feedback,
incentive-driven system. When a simple system tries to regulate a
complex system you often get unintended consequences.
Unintended consequences are not restricted to government regulation
of society but can also happen when government tries to regulate other
complex systems such as the ecosystem (e.g. fire prevention policy
that reduces forest diversity and increases mass fires, dam building
that destroys wet lands and makes floods more likely etc.) Unintended
consequences can even happen in the attempted regulation of complex
physical systems (here is a classic example involving turbulence).
Read the whole thing.
Here is my attempt to get at the connection between complexity and unintended consequences.