We know that PolitiFact is non-partisan because it doesn't make mistakes like this.
A May 22, 2018 PolitiFact article (with no "Truth-O-Meter" rating) by John Kruzel looked at allegations of a secret meeting at a Paris restaurant between former secretary of state John Kerry and Iranian representatives.
PolitiFact judged that no solid evidence supported the allegations. More interestingly, PolitiFact framed its article as a defense of Kerry from charges he violated the Logan Act.
And that's where PolitiFact slipped up. Badly.
PolitiFact (bold emphasis added):
Trump and right-wing backers challenged Kerry’s actions as violating the 18th century Logan Act, which prevents U.S. citizens from privately meeting with a foreign government to sway its decisions on matters involving the United States.PolitiFact implies that because the private restaurant meeting probably didn't take place therefore charges Kerry violated the Logan Act have no basis in fact.
The problem? The Logan Act doesn't forbid U.S. citizens from privately meeting with foreign governments to make policy agreements on behalf of the United States. The Logan act prevents private citizens from conducting U.S. foreign policy on behalf of the United States.
The Logan Act:
Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both.Making PolitiFact's fact check even more hilarious (and slanted), the paragraph preceding PolitiFact's erroneous description of the Logan Act describes Kerry meeting with various foreign officials, including an Iranian, regarding the Iran deal (bold emphasis added):
This section shall not abridge the right of a citizen to apply, himself or his agent, to any foreign government or the agents thereof for redress of any injury which he may have sustained from such government or any of its agents or subjects.
In the weeks before Trump’s May 8 decision to exit the deal and re-impose sanctions on Iran, Kerry had worked frantically behind the scenes to preserve the deal he helped craft in 2015, according to the Boston Globe. Ahead of the U.S. withdrawal, Kerry, who was secretary of state under President Barack Obama, met with Iran’s Foreign Minister Javad Zarif, courted European officials and made dozens of calls to members of Congress in hopes of salvaging the accord.PolitiFact's apparent effort to exonerate Kerry with its framing of the story ends up convicting Kerry, with the Logan Act properly understood.
How does a non-partisan fact checker make such a huge mistake?
Don't ask us.
Update May 23, 2018: Updated link to Internet Archive version of the PolitiFact article. The first version of that URL was somehow defective.