Who was the "mistake" in "The Westing Game"?Barney Northrup rented apartments in Sunset Towers to one tenant who was a "mistake" -- which one was it?

The tenant to whom Barney Northrup rented an apartment in Sunset Towers by mistake was Sydelle Pulaski.

Barney Northrup had carefully chosen whom he wanted to live at Sunset Towers.  Each of the potential tenants whom he then aggressively courted had some connection to Sam Westing.  It is not revealed until later which of the tenants was the "mistake" (Chapter 1), but throughout the Westing Game, Sydelle stuck out because no one could figure out her connection to the game's inventor.  Sandy McSanders, the doorman and one of the Westing heirs, compiled a notebook listing everything that was known about each of the players.  On Sydelle Pulaski's entry, he answered the section about her "Westing connection" with only a question mark (Chapter 18).  Later, Judge Ford uncovered a transcript of an interview with a childhood friend of Bertha Erica Crow, credited to a Sybil Pulaski (Chapter 19).

The "mistake" was caused by Otis Amber, a private investigator hired by Barney Northrup to investigate the six people originally sought as tenants for Sunset Towers.  They were supposed to be J.J. Ford, George Theodorakis, James Hoo, Gracie Windkloppel, Flora Baumbach, and Sybil Pulaski.  Sybil Pulaski was a close friend of Crow, who turned out to be Westing's ex-wife.  Amber mistakenly misread her first name, Sybil, and ended up investigating a Sydelle Pulaski instead (Chapter 26).