The Willowbrook State school was a state funded institution for children with intellectual disabilities in Staten Island, New York from 1930 until 1983. The school was known to have poor conditions and questionable intentions. In 1972, Geraldo Rivera brought cameras into the school and exposed the conditions. 

 Books (and more) that consider journalism ethics and also include significant discussion of undercover reporting.

From 1968 to present day, reporters have gone undercover to expose the corruption and mistreatment that occurs within nursing homes.

Since the 1870s, journalists have been posing as patients or attendants to expose horrid conditions and treatment inside mental hospitals. Nellie Bly, incidentally, was not the first.