Viewership data company Nielsen has used their ratings system to help advertisers negotiate the cost of TV commercials with networks for decades. However, the company reported Friday that an error in their database may have impacted several months of TV ratings, a mistake which will drastically tarnish their reputation.

Nielsen’s official statement revealed that the “technical error” was introduced on March 2, but the company did not discover it until Oct. 6, since the error was “generally imperceptible.” Only after they noticed strangely high viewing levels during fall season premiere week did the company notice the glitch which misattributed “small amounts of viewing for some national broadcast networks and syndicators.” While cable networks and local TV stations were not impacted, major broadcasters such as those under Disney, ABC, Comcast, and the national networks of CBS Corp and Twenty-First Century Fox were all affected.

Upon discovering the data error, Nielsen immediately deployed software to fix the problem. Their “exhaustive post-mortem” efforts will ideally correct their ratings and attempt to determine the true viewership values of this season’s shows.

“In the vast majority of cases, the impact is small; in a handful of cases, the impact is more significant,” states the company.

However, even if most programming only sees a difference of 0.05 of a ratings point, that mere tenth of a ratings point can still largely affect advertising rates.

“This is the equivalent of an earthquake. When you can’t trust the ground you’re standing on, it’s disorienting,” said a network insider. “What else has gone wrong that we don’t know about?”

“Our entire industry relies upon Nielsen for accuracy and veracity, and we hope that they can quickly resolve this issue,” wrote powerhouse network ABC in a statement Friday. “We’re confident that the momentum we’ve seen across the network so far this season will continue.”