THE KILLER CARD
Analog telephone networks sling around audio tones at a number of frequencies, which used to have an annoying way of leaking out of trader loudspeakers, disrupting life. The Killer Card, installed across an incoming circuit, listened for these tones, typically 1004, 2600 and 2713 Hertz (if you've read this far you may even know what they're for!), disconnecting the circuit until the tone stopped, and sending a latching alarm signal. A visiting AT&T engineer said, 'Why didn't we think of that?'