And then there is the case of Fnu Lnu.

In one of the quirkier aspects of the legal system, when suspects are charged but their identity remains a mystery, they are often listed in court records as “Fnu Lnu,” shorthand for “First name unknown, Last name unknown.”

The letters, often pronounced phonetically (“Fuhnoo Luhnoo”), may refer to a person caught up in an immigration sweep or overheard on a wiretap. At any given time, there may be hundreds of Fnu Lnus in the courts.

But the acronym can spark confusion when mistaken for a genuine name. In the 1990s, a Newport News, Va., newspaper article listed “Fnu Lnu” as if that were an actual defendant charged in a drug and murder case. The resulting correction ended up being seen by the playwright Mac Wellman, who wrote a play called “Fnu Lnu” that was produced in 1997 by Soho Rep in an Off Broadway theater.

In Manhattan, a bedeviling Fnu Lnu case came to a conclusion just a few weeks ago. A man claiming he was “Hernan Sanchez” was charged with making a false statement in a passport application, but records suggested he had used at least six identities.

When the man agreed to plead guilty in January, the prosecutor, Ryan Finkel, told Judge Gregory H. Woods that the man’s identity was still uncertain — “a question mark,” Mr. Finkel said.