Since falsely reporting one of the indicators of the possible efficacy of their ads over the span of a year, Facebook (FB) is paying back some advertisers.

Between August 2019 and August 2020, the corporation’s “conversion lift” tool experienced a glitch that allegedly affected thousands of advertisements.

In September, Facebook patched the mistake and is now providing a loan to consumers “significantly affected” by the error.

According to an explanation of the tool on Facebook’s website, conversion lift lets involves systematically how advertisements contribute to revenue, using a “gold-standard methodology” that connects advertising on Facebook sites like Instagram, to market results.

The free tool presents advertising for different evaluation and control categories and then compares each revenue conversion. Then an advertiser will determine how much to invest in the social network depending on the outcome of the analysis.

But marketers, according to a post on the AdExchanger website, were only alerted to the mistake this month.

We found a technological error that impaired some conversion lift testing when making changes to our measuring goods. We have resolved this and are partnering with advertisers that have affected studies. A “tiny proportion” of advertisers were affected, and according to the firm, they would get a “one-time credit.”

It’s not the first time that Facebook has confessed to reporting errors. In September 2016, it said it misjudged the total time users spend watching video ads over a two-year period, and a study showed in 2017 that in some U.S. states and counties, Facebook appeared to attract more people than official demographic figures said existed in those regions.

Facebook is a global platform for ads, whose revenue comes mainly from smaller advertisers. In the q3, its ad sales hit $21.2 billion, up 22 percent on the year before and it expects holiday demand in the 4th quarter to boost ad revenue.