Inadvertent opening of the lateral semicircular canal during mastoid surgery is, fortunately, a rare injury. Accidental fenestration of the bony semicircular canal often results in profound hearing loss and severe vestibular damage. In the past the lateral canal was most at risk, but with the introduction of endolymphatic sac surgery and middle fossa procedures, the posterior and superior canals are now also in danger. The mechanism of cochlear protection after the trauma remains obscure. Recently the authors experienced a case of accidental surgical fenestration of labyrinthine fistula in chronic otitis media with cholesteatoma.