What were the negative effects experienced by participants in the Zimbardo prison experiment?

The American educational network PBS did an Excellent series in 2001 called "Discovering Psychology," and in episodes 19-20, it addressed such things as the psychological effect the situation can have upon how human beings act. In one segment, it discussed the 1971 Prison experiment, with Dr. Zimbardo (host of the series) looking back on it. It also showed a follow up, where students (both the "guards" and the "prisoners") met after a few months and talked about their feelings regarding the experiment.

It is clear from that segment that the participants were still upset. The "prisoners" were shocked that classmates they thought they knew were willing to treat them so brutally, and the "guards" were still embarrassed that they had gotten so caught up in the role that they acted in such an inhumane manner. Even Dr. Zimbardo noted that he too had become caught up in his role of the "warden," and his reaction surprised him. Although he was a trained psychologist, he admitted he had lost perspective and found himself thinking like a prison warden. That was one reason he called off the experiment-- the realization that it does not take much (in this case, some isolation, uniforms, and arbitrary hierarchies of power) to make otherwise decent people behave in unethical or cruel ways.