Many Twitter users noticed that “Kill All Jews” was among the platform’s trending topics today.

Twitter has since removed the hateful phrase from its trending sidebar. A spokesperson told Motherboard that it “should not have appeared in trends, and we’re sorry for this mistake.”

Trending items are algorithmically surfaced, so if a hashtag or, say, celebrity name is spiking on Twitter, it may show up in your sidebar. Trends are “tailored for you based on who you follow, your interests, and your location,” according to Twitter’s FAQ.

“At times, we do prevent certain content from trending and we have now done so with this trend,” the Twitter spokesperson said. “This was trending as a result of coverage and horrified reactions to the vandalism against a synagogue in New York. Regardless, it should not have appeared as a trend.”

On Thursday night Brooklyn’s Union Temple synagogue was vandalized with graffiti that said “Kill All Jews,” ahead of an event featuring Broad City actress Ilana Glazer, journalist Amy Goodman, and New York state senate candidates Andrew Gounardes and Jim Gaughran. The event was canceled for security reasons. The vandalism occurred less than a week after the deadliest anti-Semitic attack in US history, in which 11 people were murdered in Pittsburgh’s Tree of Life Congregation synagogue.

Today’s mistake underscores how Twitter can passively become a platform where racist ideologies are easily spread. CEO Jack Dorsey recently disputed Twitter’s reputation as “the free speech wing of the free speech party,” which is what Twitter’s then-general manager called the website in 2012. But Twitter’s public stance on free expression, along with its nebulous guidelines, have been used to justify a litany of hate speech.

Roughly 4.2 million anti-Semitic tweets were shared or retweeted by 3 million Twitter accounts last year, according to an analysis by the Anti-Defamation League. The platform also seems to have trouble catching coded hate speech, such as the word “termite,” which can be used in an anti-Semitic context.

Social media platforms like Twitter and Facebook have praised algorithms for being able to detect abuse, but it’s clear that AI isn’t a comprehensive solution for combatting social media’s most insidious problems.