From Bruce Schneier’s Crypto-Gram of 15 April 2004:
Here’s a story of a woman who posts an ad requesting a nanny. When a potential nanny responds, she asks for references for a background check. Then she places another ad, using the reference material as a fake identity. She gets a job with the good references—they’re real, although for another person—and then robs the family who hires her. And then she repeats the process.
Look what’s going on here. She inserts herself in the middle of a communication between the real nanny and the real employer, pretending to be one to the other. The nanny sends her references to someone she assumes to be a potential employer, not realizing that it is a criminal. The employer receives the references and checks them, not realizing that they don’t actually belong to the person who is sending them.