Holodeck
In the fictional Star Trek universe, the holodeck is a form of virtual reality. It is an enclosed room in which objects and people are simulated by a combination of replicated matter animated with weak tractor beams and shaped force fields onto which holographic images are projected. The effect is a simulation of entire environments, with which the user can interact. Two of the main purposes of the holodeck are recreation and training. They are also sometimes used to recreate a crime or questionable incident to determine the forensics and logistics of it for law enforcement purposes.
The holodeck includes safeguards known as security protocol designed to protect the safety of the user or users.
Despite the frequency at which holodecks have malfunctioned (as a plot device, holodeck malfunction is right up there with transporter malfunction), Starfleet has apparently not taken the initiative to disable and remove them.