What potential “harm” could occur to a research subject?

This is an evidenced based research class for nursing. Please use the available link as a resource to answer this question. Its a discussion question so I don’t need anything long as an answer either.

http://www.american.edu/irb/IRB-Training.cfm

What potential “harm” could occur to a research subject? How does a researcher minimize harm? What should a researcher do if harm does happen?