[A*O]

My cousin is an anaesthetist and he won't officially admit it happens, but unoffically he says it does. Usually because the patient has an underlying medical condition that makes the drugs less effective. However, as sputnik says you may be 'aware' of what's happening but it's highly unusual for you to feel actual pain. Among the drugs they give you are powerful muscle relaxants so you are paralysed during the proceeding which is the scary part.