Intra-operative Awareness (a.k.a. Anesthesia awareness) sometimes occurs as a complication during general anesthesia, where the intended state of complete unconsciousness is not maintained throughout the whole procedure.

The incidence of this anesthesia complication is variable and seems to affect 0.2% to 0.4% of patients according to the surgical setting carried out. This variation reflects the surgical setting as well as the physiological state of the patient. Thus, the incidence is 0.2% in general surgery, about 0.4% during caesarean section, between 1 and 2% during cardiac surgery and between 10% and 40% for anesthesia of the traumatized."

Source : Wikipedia

The causes of intraoperative awareness are as yet unknown, and the problem may be multifactorial. At least 4 broad categories of causes are plausible. First, unexpected patient-specific variability in dose requirements of anesthetic drugs may be a result of altered expression or function of target receptors. Second, patients may be unable to tolerate a sufficient dose of anesthetic because of low physiologic reserves related to factors such as poor cardiac function or severe hypovolemia. Third, physiologic characteristics that would indicate the need for a dose change may be masked by factors such as use of β-adrenergic receptor blockers or presence of a pacemaker. Fourth, intended drug delivery systems may be compromised by events such as equipment malfunction or misuse."

Source : Awareness during anesthesia Beverley A. Orser, C. David Mazer and Andrew J. Baker, CMAJ January 15, 2008 178 (2) 185-188.

Also see :General Anaestheticsplugin-autotooltip__plain plugin-autotooltip_bigGeneral Anaesthetics

A general anaesthetic is a drug that brings about a reversible loss of consciousness.* [ see note below ]

In formal use since 1842 - the first public demonstration used Diethyl ether to carry out surgery. (Though it's likely that alcohol, another chemical classed as an anaesthetic, was previously used, extensively, for similar purposes).