What is commonly referred to as "the forbidden experiment" in the research of language acquisition is the use of feral children to conduct isolation experiments involving leaving a child incommunicado in order to see what happens in the language acquisition process.
It was common practice centuries ago to assume that language proceeded either from nature, from divinity, or from another weird source that would require taking the child in complete physical isolation. The idea was that kids would speak their "natural" language if they are not exposed to any deterrent.
It is said that a Russian tsar raised feral children to learn what language God would motivate them to speak. It is known that all children who lack communication and are completely isolated die during childhood. Hence, continuing with the experiment of isolation to research language is equivalent to pre-meditated murder, kidnapping, and lots of anti-ethical behavior.