In or around 2001, a 28-year-old named Begoña (her last name has gone unreported) walked into a store at a mall in the Canary Islands. She was there to do something many young adults do -- she wanted to buy herself some clothes. From her perspective, the entire transaction was without incident: she found the shirt she wanted, paid the store clerk, and went home. A few days later, she decided she didn't want the shirt after all. So she returned to the store to exchange it -- and found out that the store clerk wasn't happy with her. When Begoña first purchased the shirt, the clerk had recognized her and said hi, but Begoña didn't return the gesture. In fact, Begoña acted as if she didn't know the clerk at all.

That moment would have come and gone without another thought but for a quirk of fate -- Begoña didn't like the shirt. As the New York Post reported, she returned to the mall to return it, and the same store clerk was there again. This time, though, the store clerk explained the confusion; she was friends with Begoña's twin sister and had assumed that Begoña was her friend, not her friend's twin. But this was weird for two reasons. First, the clerk didn't previously know that her friend had a twin. Second, while Begoña did have a twin sister, Beatriz, a mix-up of this variety seemed likely --  Begoña and Beatriz were fraternal twins and didn't resemble each other all that much.

The clerk, as it turned out, wasn't talking about Beatriz, though; she was speaking about a third woman named Delia. Everyone was confused but intrigued. According to Nancy Segal, a professor of psychology who focuses on the relationships of twins, "a meeting was arranged for that evening, and it turns out that these two twins who had been separated for 28 years realized that they really belonged together. DNA tests followed, [and] the twin-ship was confirmed." Delia and Beatriz had been switched at birth.

Segal, a twin herself, focuses much of her research on situations like this; her 2011 book on the subject, "Someone Else's Twin: The True Story of Babies Switched at Birth," focuses on the extremely rare but shockingly not unheard of cases like this. As Segal wrote in Psychology Today, "only seven or eight switched-at-birth twin cases have been documented," and all have involved identical twins. And they don't usually have happy endings. Segal interviewed Begoña, Beatriz, and Delia, and found that they all had a hard time coming to terms with the switch. None of the three had much of a relationship with their biological-but-absent relatives even after learning about the switch (Begoña, who lived with her biological parents the whole time, has a limited relationship with Delia). Beatriz, in particular, had a difficult time relating to her non-biological mother after the discovery; per Segal, Beatriz felt that it took a while for her mother "to accept the fact that one of her twin daughters had been accidentally exchanged with another baby." On the plus side, the discovery solved some minor mysteries for the three girls; Beatriz and Begoña now understood why they didn't have the connection that other twins had. And per Segal's Psychology Today article, "when Delia became ill with cancer as a teenager, it turned out that neither her parents nor her two younger sisters were compatible bone marrow donors." Now, she knew why.

As for the hospital that caused the mix-up, they ended up with a pricey legal bill in the end. According to the University of Chicago's magazine, in 2009, a court "awarded the families €900,000 (then about $1.2 million)."