This happened to my roomate and housemate. We were living in a two family house and both apartments were young, single girls. One night, two of them had dates – lets call them Chani and Rivki. The Rivki left with her date. The next guy showed up and him and Chani realized that this guy was supposed to go out with Rivki! They called up Rivki, who came back and swapped dates.
Chani and her proper date ended up getting married.