We present an unusual source of oversensing following an internal cardioverter-defibrillator generator change. The early appearance of reproducible myopotentials in the defibrillator sensing channel is usually due to a technical complication at the time of device implantation. Clues such as abrupt impedance change or reproduction with mechanical stimulation can help to localize a problem. Frequently the complication requires reoperation to examine the system. What do you do when everything seems to be working fine?