The 4400

The first season of The 4400 featured "unintended consequences" is every episode.  While this theme continued in future seasons, the first is the best for contemplation of this important concept.

During the past century, thousands of people have gone missing. When 4400 of them return all at once unharmed and looking the same as when they disappeared, the government investigates, unsure of how this can be possible. What the government does not know is that the presence of these 4400 will change the human race in many unexpected ways.