“Any one of those three things could come up at any time,” he added, “but for those to hit you all at the same time, wow.”

Network shows are judged by small fractions of ratings points and perceptions of a show’s success or failure are often determined by whether the show gained or lost as little as a tenth of a point. In one example that will surely be raised, ABC News made headlines this last week by surpassing NBC News’s evening broadcast for the first time in six years. NBC will undoubtedly question those results now, especially because it has noted for months that ABC began closing the ratings gap in April — or one month after the pro-ABC error affected Nielsen’s system.

Even if the ABC gains were entirely legitimate, they now have a shadow over them. ABC issued a statement on Friday saying that despite the error, the network was confident that it would maintain the ratings momentum that its programming has seen in the opening weeks of the new television season.

The Nielsen executives Pat McDonough and Steve Hasker said repeatedly in their news conference on Friday that the incorrect ratings — which had affected every program on ABC, not just the ones in prime time — fell “well within the tolerance of statistical error.” They said any changes in numbers or the rankings of programs would be largely insignificant and would be corrected when Nielsen issues new ratings on Monday.

But in a statement sent to clients, the company said, “In the vast majority of cases the impact is small, but in a handful of cases the impact is more material.”

The Nielsen executives emphasized that ABC had nothing to do with the incorrect ratings and blamed a new software program that was introduced in March. The mistakes affected something called “all other tuning,” an arcane part of the measurement of broadcast ratings (cable network ratings are unaffected).

Mr. Hasker said in the phone call on Friday that the company had first detected the discrepancy itself, a point challenged by several executives at the broadcast networks.