In chapter 19, J. J. Ford reads Otis Amber's report, which happens to be a transcript from an interview with Berthe Erica Crow’s childhood friend, Sybil Pulaski. According to the transcript, Sybil Pulaski grew up with Sam Westing's former wife and considered Berthe Erica Crow to be her best friend. After reading the transcript, the judge notices that Sam Westing wanted Sybil Pulaski, Berthe Erica Crow’s childhood friend, to be an heir instead of Sydelle Pulaski. Therefore, Sydelle Pulaski is the mistake in Westing's game and was never supposed to be a tenant in Sunset Towers in the first place.

Sam Westing, under the guise of Barney Northrup, planned to invite sixteen specific people to play his complicated game with the chance of inheriting his wealth. Sydelle is the only tenant with no connection to Sam Westing, which contributes to the mystery of the story.

Throughout the story, Sydelle Pulaski is portrayed as an eccentric hypochondriac who is often overlooked and desperately in need of attention. Sydelle is a secretary and becomes partners with the attractive Angela Wexler in Sam Westing's game. She is the only heir with the presence of mind to take handwritten notes of Sam Westing's will when it is read aloud, and she cleverly writes her notes in Polish to prevent the other heirs from comprehending the will. Without Sydelle's copy of the will, Turtle might not have been able to solve the mystery.

Sydelle Pulaski and Sybil Pulaski may have very similar names, but in terms of personality, they're streets apart. Whereas Sybil is a very sweet lady, a friend of Barney's wife since childhood, Sydelle is a crusty old spinster with a streak of exhibitionism. Barney—also known as Sam Westing—makes an understandable mistake in getting their names mixed up. Instead of renting out the spare apartment in Sunset Towers to Sybil, as he originally planned, he accidentally rents it out to Sydelle.

Though technically speaking, Sydelle has no connection to Sam Westing and therefore shouldn't be part of the Westing Game, she nonetheless shows herself to be rather adept at playing it, ending up five thousand dollars richer for her participation.

Despite being somewhat annoying, Sydelle does at least have the saving grace of being incredibly bright. We can observe this when she shows herself to be instrumental in solving the problem of the will. Sydelle, who's a secretary, has the presence of mind to take down notes on the will the first time it's read through. She writes these notes in her native Polish, making it virtually impossible for anyone else to read them.

These notes prove to be particularly handy during the trial scene, when they will become instrumental in solving the mystery of Sam Westing's will.

Chapter 1 tells readers about various people being delivered a letter by a sixty-two-year-old delivery boy. The letters are from Barney Northup. We are also told that there is no such person as Northup. Each letter invites the recipient or recipients to live in the Sunset Towers. The chapter ends with telling readers that one of the tenants was invited by mistake.

And, oh yes, one was a bookie, one was a burglar, one was a bomber, and one was a mistake. Barney Northrup had rented one of the apartments to the wrong person.

Readers are given a hint that the unintended tenant might be Sydelle Pulaski in chapters 18 and 19. By this point, readers are likely to be very curious about her anyway, based on her quirky habits; however, it isn't until chapter 26 that readers are told exactly how and why Sydelle is at the Sunset Towers and why she technically shouldn't be. Otis Amber admits that he was hired by Northup to investigate six specific people. He was supposed to investigate Ford, Theodorakis, Hoo, Windkloppel, Baumbach, and Pulaski. Amber admits that he later discovered that it was Sybil Pulaski he was supposed to investigate. That would have changed who got Northup's invitation.

Judge J. J. Ford, George Theodorakis, James Hoo, Gracie Windkloppel, Flora Baumbach, and Sybil Pulaski. I made a mistake on the last one; I wasn’t aware of the mix-up until I looked into Crow’s early life for the judge. It seems I confused a Sybil Pulaski with a Sydelle Pulaski.

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).