On Friday morning, a group of executives at online electronics retailer Newegg Inc. began their daily “TMZ,” a conference call named after the celebrity-news TV show, on which participants discuss issues that arose during the night on social media.

The social-media team alerted the group that several people had sent Twitter messages to Newegg asking why the company was advertising on Breitbart News Network, a site known for scorched-earth populism that is popular with the “alt-right,” a loose ​conglomerate of ​far-right groups that embrace tenets of white supremacy or say they reject mainstream conservatism.

During the meeting, a Twitter message was read aloud saying: “Your ads are embedded in hate-spreading, white nationalist Breitbart ‘news’ articles. Pls consider opting out,” a person at the company familiar with the conference call said.

Newegg didn’t know its ads were on Breitbart, the person said, adding that the company quickly discovered the ads got there as part of an automated ad buy Newegg had placed with Alphabet Inc.’s Google, which helps marketers place advertisements across thousands of websites.

Newegg has since added Breitbart News to a list of websites on which it doesn’t want its ads to appear, the person said. Such a move is known in the industry as blacklisting. “Newegg doesn’t have any political favorites and wouldn’t advertise on any politically charged site,” Merle McIntosh, Newegg’s senior vice president of sales and marketing, said in a statement.

A spokeswoman for Google said the company doesn’t comment on its relationships with specific business partners.

Several other advertisers say they have pulled advertising from Breitbart following mounting pressure from people using social media, including foods company Kellogg Co. , insurer Allstate and eyewear company Warby Parker. Others, including grooming appliance company Braun have also posted statements on Twitter saying they have ceased advertising on Breitbart.

Allstate said in a tweet that it would exclude Breitbart News from its automated ad buys in the future. A company spokeswoman declined to comment further.

A Warby Parker spokeswoman said the company is now actively blocking its ads from appearing on Brietbart. The eyewear company said its ads appeared on the site via a third-party advertising company it works with.

Breitbart editor in chief Alexander Marlow on Friday said there has been “little to no impact” on the site’s advertising revenue since some advertisers announced they were pulling their ads.

“We’ve had a hundred or so companies approach us in the past few days and say they want to advertise with us specifically because they do share our values,” Mr. Marlow said.

Traffic to the site is currently at an all-time high, Mr. Marlow added.

Regarding the tweet read aloud at the Newegg meeting, Breitbart chief executive Larry Solov said, “We’re not a white nationalist site. Anyone who would believe that based on a tweet is showing blatant contempt for our 45 million readers and the mainstream American values we hold dear.”

The conservative news website has taken aim specifically at Kellogg, on Wednesday called on its readers to boycott Kellogg products, which include Frosted Flakes and Froot Loops.

Breitbart describes itself as “the largest platform for pro-family content” on the web, and adds, “We advocate for traditional American values, perhaps most important among them is freedom of speech.”

Kellogg this week said it works with its media-buying partners to ensure its ads don’t appear on sites that aren’t “aligned with its values,” relying on filtering technology to detect webpages on which Kellogg wouldn’t want to place ads. “As you can imagine, there is a very large volume of websites, so occasionally something is inadvertently missed.”

Digiday earlier wrote about marketers removing ads from Breitbart.

The situation at Breitbart is bringing into sharp focus the complications of ad technology and the risks for marketers. Using sophisticated online ad-buying technologies, many advertisers now pay to place ads in front of specific users as they move around the web, as opposed to placing ads on specific websites.

This approach can be efficient and help marketers target desired audiences across thousands of sites. But the strategy presents risks, as ads can sometimes appear in some unintended places, unless advertisers proactively prevent from appearing there.

Advertising showing up on unintended websites “raises yet again questions about the lack of controls there really are with programmatic ad buying,” said one major ad buyer.

According to ad agency executives, unless advertisers specify which on sites their automated ads should appear, there isn’t a fail-safe way to stop ads from appearing on certain sites. Online ad companies sometimes disguise or “mask” the websites on which ads will actually appear. But if marketers only work with a preapproved list of sites, they risk limiting the scale and performance of their ad campaigns.

Advertising technology company AppNexus last month said it wouldn’t allow its tools to be used to buy and place ads on the Breitbart site. But other ad technology companies are allowing their technology to place ads on Breitbart.

Ad tech company Criteo has also been enabling marketers to place ads on the Breitbart site in recent days, but declined to comment on specific cases. “We are committed to an environment that protects the brand integrity of our clients” and have in place a process to enable advertisers to block ads from appearing on certain sites, the company said.

Taboola, a company that helps publishers sell ads that recommend other web articles, places advertising on Breitbart. Taboola’s chief executive Adam Singolda, declined to comment about Breitbart, but said in an email that he believes “it is not Taboola’s job, nor place, to censor or dictate an opinion as to what is allowed and what is not allowed for people to write about.”

The Breitbart situation is a high-profile example of online ads showing up in unintended places, but it isn’t a new problem. Ads from major brands frequently show up on websites promoting pirated content, for example.

Spotting an opportunity, scores of ad technology companies now offer products they say can ensure marketers’ ads only appear on “brand-safe” webpages. Meanwhile, many advertisers say they are actively seeking greater transparency from their ad tech providers to help combat the issue.

Write to Jack Marshall at Jack.Marshall@wsj.com and Suzanne Vranica at suzanne.vranica@wsj.com