SAN FRANCISCO — Custom T-shirt design company Teespring is once again on the defensive after a media group found it was selling a T-shirt about lynching journalists on its site and as a third party seller on Walmart.com.

Walmart was alerted to the shirt's presence on its site on Wednesday by the Radio Television Digital News Association and that same day removed it from sale. The T-shirt read "Rope. Tree. Journalist. SOME ASSEMBLY REQUIRED”.

The discovery highlights the ongoing problems created by technology companies that largely lean on software to screen out harmful or abusive content. These automated systems are cost effective, underwriting healthy profit margins, but have made Internet companies — including Facebook and Google — vulnerable to individuals that figure out how to skirt the system for harm.

"Until there’s an economic cost for companies that are doing moderation poorly, there’s not a really an incentive to do it well,” said Libby Hemphill, a professor of information studies at the University of Michigan in Ann Arbor, Mich.

Teespring is a San Francisco-based company that has raised millions of dollars from Silicon Valley sources such as start-up incubator Y Combinator and venture capital firms including Andreessen Horowitz and Khosla Ventures.

Its business model is to act as an intermediary: Customers upload designs for custom T-shirts and other logo items. They then sell the items either on the Teespring site or on their own sites. Teespring takes a cut of all sales from users.

Multiple examples of inappropriate material have dogged the company. Teespring was in the news in May after shirts saying “Black Women are Trash” were discovered for sale on the site.

In a statement to USA TODAY, Walmart said the item was sold by a third-party seller on its marketplace and clearly violates its policies. "We removed it as soon as it was brought to our attention, and are conducting a thorough review of the seller’s assortment," the statement read.

In this instance, an individual created the T-shirt on the Teespring site, which was then automatically aggregated to Walmart's site.

Walmart has moderation policies in place to vet material sold on its site but did not immediately respond to questions about how the system functioned or how the T-shirt was not picked up by those systems.

Teespring's acceptable use policy bars racist and other offensive content as well as content that promotes or glorifies harm or violence to individuals. According to the company it monitors for such content by using a combination of automation and human review.

In the May incident, Teespring blamed new computer code pushed out by its engineering team that tagged the slogans as offensive but failed to remove them from the store as the company's policies dictated, according to a statement send to USA TODAY at the time.

More: Teespring blames code, human error for 'Black women are trash' T-shirts

In a statement to USA TODAY on Thursday, Teespring did not explain why the T-shirt was able to slip through its systems, which include an image recognition scan by a San Francisco-based company called Pavlov.

The company said it also has a human team based in Hebron, Ky. that checks for items the automated sites don’t find.

“While a very small amount of problematic content does still slip through our filters, we are working hard to close those gaps and continue to improve our systems,” the statement read.

However, inappropriate content continues to appear on the site, a USA TODAY analysis found.

Shirts reading “Hitler Did Nothing Wrong” are currently for sale on the site, as is a T-shirt featuring an image of Bill Cosby on a page titled "Party Time with Bill Cosby" featuring a slogan that reads, "drinks on me ladies," a reference to accusations against the television star that he drugged and raped women.

In October a shirt reading, “Eat Sleep Rape Repeat” was also available on Teespring. It was later removed from the site

The slogan found on the journalism shirt has been around for several years. During the 2016 presidential election cycle it was spotted at campaign rallies and in public areas around the nation.

Moderating potentially inappropriate content is an ongoing issue for many Internet companies that rely on user-generated content. None thus far have come up with good solutions, say experts who study the problem.

“I don’t think these sites really want moderation. I don’t think they care,” said Kishonna Gray, a professor of communication at Arizona State University who studies online moderation issues. "They don’t want to disturb their profit margins."