In early June, employees at the digital car marketplace Autotrader UK discovered that they had a problem.

Dozens of site visitors were complaining that they were unable to see car listings supplied by site’s car dealers, rendering the site difficult, if not impossible, to use.

After some digging, Autotrader UK figured out that the malfunction was only happening to visitors using ad-blocking software, which helped lead staffers to the source of the problem: EasyList, an open-source, community-run list of rules that powers popular ad-blocking software including Adblock Plus and the browser Brave.

The crowdsourced list helps control the browsing experience of a substantial number of internet users. Though EasyList does not publish current figures about its use, Adblock Plus alone has been downloaded more than 100 million times. UBlock Origin, another popular adblocker, has more than 15 million active users across Chrome and Firefox.

While the purpose of the list has always been to keep advertising out of web experiences, EasyList’s rules regularly break normal editorial features on sites. In the past six months, EasyList changes have broken the buy buttons on commerce site The Inventory, the video player on Animal Planet, disrupted site navigation on Fandom, and disrupted the style and CSS loading process on job search site Indeed.

Though most of these issues were resolved quicklyas publisher sites continue to evolve, they have to contend with the possibility that they might run afoul of one of the most important crowd-maintained documents on the internet. EasyList did not respond to a request for comment.

“It’s crazy that more people don’t know about this,” said Marty Kratky-Katz, the founder of Blockthrough, which lets publishers monetize using Adblock Plus’s Acceptable Ads program. “I don’t think they mean any harm or have any malicious intentions. But it’s not like they were experts in balancing publisher monetization, or like they were elected. They’re just four dudes.”

EasyList was originally launched in 2005, as a kind of add-on to the Adblock browser extension. Several different people have overseen it since then, and today, a group of four people, led by a man named Ryan Brown, is authorized to change EasyList’s rules.

Over time, its list of rules (and exceptions to those rules) has grown sprawling. Analysis conducted by Brave last summer found over 70,000 rules in EasyList, a mixture of network rules, which determine whether a site fetches sites or code from web addresses that match a certain kind of pattern; element rules, which dictate whether certain page elements, such as banners, can be displayed; and exceptions to the element and network rules.

The exceptions, which constitute about 9% of EasyList’s rules, according to Brave, are necessary because EasyList’s network and element rules sometimes break a normal site’s functions. In many cases, the breaks are caused when developers use a word in their code that EasyList has banned, such as “advert.” Those sorts of issues are easiest to fix by site developers. “Sometimes it’s easier just to change things on our end,” said Josh Butts, svp of product and technology at Ziff Media Group, which operates a portfolio of commerce, shopping and editorial sites.

But there are occasions when those changes are hard to make. In Autotrader UK’s case, for example, the change would have involved fixing hundreds of thousands of pages. That compelled them to visit EasyList’s message board, where it had to plead its case and hope an EasyList author would decide to make a change.

In many cases, fixes are minor and people from EasyList make them quickly, often within a matter of hours. But sometimes its authors dig in. The Autotrader UK example took nearly a week to resolve, in part because one of the list’s authors made it clear that he would prefer Autotrader UK to change code on their end, rather than write an exception.

Autotrader’s parent company did not respond to a request for comment.

Those periodic disruptions are trouble for both parties. But ad-blockers don’t mind. “Is there stuff they’re blocking that maybe they shouldn’t? Yes,” Kratky-Katz said. “But I think it’s a price worth paying if you’re in favor of ad-blocking.”

An earlier version of this story confused uBlock and uBlock Origin. uBlock is owned by Adblock Plus; uBlock Origin is independent.