Strings of them have a plug on one side and a receptacle on the other. When hanging the lights, people don't notice if they have someone backwards and end up with a receptacle facing a receptacle. Rather than do the smart thing and turn the lights around, they go to the hardware store for assistance in killing themselves.
Another common one is backup generators.
Someone will turn off the breaker in the service panel, then plug a suicide cord from any receptacle in the circuit, to the output of a portable generator.