This story describes a 1966 journal article that argues that signing an informed consent isn't the same as truly giving informed consent.
I think this is a good example for the ethics section of a research methods class as it demonstrates some deeply unethical situations in which participants weren't able to give informed consent (prisoners, non-English speakers, etc.). Indeed, the context within which the informed consent is provided is very important. It also provides a historical context regarding the creation of Institutional Review Boards.
The original 1966 article is here.