Electronic communication can easily be hacked and eavesdropped on, it happens all of the time either on the target computer itself or via communication links. Typewriters of course can't be hacked... well actually they can and were during the Cold War!
The Soviet Union developed a sophisticated listening device which were fitted inside IBM Selectric typewriters at the US embassy in Moscow and consulate in Leningrad. The listening device was almost undetectable when fitted and measured the movements of the print ball in the typewriter using magnetometers before sending it's findings via radio. The Soviets were therefore able to know, with a decent amount of accuracy and a little bit of statistical analysis, what had been typed on the machine.
The Americans discovered the device after being tipped off by the French who has discovered a similar device in a teleprinter. They began an audit of all of the office equipment in the embassy, they had also discovered a mysterious antennae hidden inside the embassy but didn't know what was it was for until they discovered the typewriter devices (and therefore were able to put 2 and 2 together).
The devices were found in seventeen Selectrics. This was a sophisticated bugging device though did rely on physical access to the typewriters to fit the device and depended on a specific design feature of the Selectric, this bug would not work on a standard mechanical typewriter.
