 The low-voltage integrated circuit inside the cartridge is extremely ESD sensitive, and can be destroyed by an event of only a few dozen volts which cannot even be felt by the person, delivering a fatal shock to the game unit.This phenomenon was described in detail by John Elder Robison (a former Milton Bradley engineer) in his book "Look Me in the Eye"; Robinson described the issue as having been a significant enough issue during the 1979 holiday season (with up to 60% of units being returned as defective) that it resulted in significant panic among Milton Bradley staff and required extensive modifications to both later Microvision units and Microvision factories (the former being of his own design) to better dispel stray static charges.The Microvision unit had a twelve-button keypad, with the switches buried under a thick layer of flexible plastic.