You’d think that the head of a $178 billion company would have better things to do than testify against a man who once pleaded guilty to possessing hallucinogenic mushrooms and was accused of defrauding customers through a now-defunct wood pellet manufacturing business. But that’s the beauty of the American justice system; when the courts come calling, even Mark Zuckerberg has to answer.

And the Facebook CEO’s time on the stand is sure to be entertaining, considering the odd set of circumstances that will have led him there.

It started back in 2003, when Paul Ceglia, the defendant in the case, posted a Craigslist ad seeking a programmer to help him develop a website called StreetFax.com. Zuckerberg, then a student at Harvard, answered the ad and Ceglia paid him $1,000 for his services.

The next time Zuckerberg heard from Ceglia was in June 2010, when Ceglia, who has on different occasions been referred to as a “software developer,” “teacher,” and “home heating entrepreneur,” filed a suit in Buffalo federal court claiming that he owned 84% of Facebook. Ceglia’s main argument was that the $1,000 he’d paid Zuckerberg in exchange for his programming help served as startup money for Zuckerberg’s planned social network.

From the very beginning, Facebook denied the allegations and called into the question the authenticity of the contract that Ceglia used as the basis for his claim.

More than a year after Ceglia sued Zuckerberg and Facebook, a judge in Buffalo—against Ceglia’s wishes—allowed the social network’s lawyers to run forensic tests on Ceglia’s computer, hard drive, and electronic media. The search turned up the actual contract between Ceglia and Zuckerberg, which didn’t mention Facebook at all. It also detected digital fingerprints left by “six USB or other removable storage devices” that Facebook’s lawyers said contained relevant documents, like one titled “Zuckerberg Contract page1.tif” and a folder labeled “Facebook Files.”

Facebook claimed that the documents were part of a trail of evidence pointing to Ceglia’s attempts to fabricate a contract in which Zuckerberg allotted him a stake in an infant Facebook.

Those unearthed files ended up serving as a smoking gun that not only led to the dismissal of Ceglia’s case against Facebook, but also prompted federal authorities to arrest Ceglia in Wellsville, N.Y., near Buffalo, in October 2012 on charges that he concocted a multibillion dollar scheme to defraud the social network. Ceglia pleaded not guilty to the charges and tried to halt the criminal case by filing another lawsuit, this one against U.S. Attorney Preet Bharara and U.S. Attorney General Eric Holder for allegedly violating Ceglia’s constitutional right to bring a civil case against Facebook. (A judge rejected that claim in March.)

As part of his criminal defense, Ceglia tried to subpoena Zuckerberg, Facebook, and Harvard for emails and documents, including disciplinary records from Zuckerberg’s time as a college student. A judge dismissed that request at a hearing on Tuesday after a prosecutor argued that the request was simply an attempt to embarrass the Facebook CEO.

It was at that same hearing that the prosecutor announced that he will call Zuckerberg as a witness in the fraud case, setting up a showdown between Zuckerberg and his Buffalo-based legal arch nemesis.

If you are looking for one more reason not to trust a random person you meet on Craigslist, this is it. Take it from Zuck: they can come back to haunt you.