Facebook was aware of inaccuracies in the way it measured how many people viewed video on its site for a year longer than it has previously admitted, court documents have claimed.
Newly released papers that are part of a US legal action against the social network, claim that it knew about the problems in 2015.
Facebook described the case against it as "without merit".
It said suggestions it had tried to hide the issue were "false".
In September 2016, Facebook admitted that it had overestimated how much video people had watched for the previous two years.
"We told our customers about the error when we discovered it - and updated our help centre to explain the issue," it said in a statement.
The error affected a Facebook metric called "average duration of video viewed", which was supposed to tell publishers for how long, on average, people had watched a video.
However, the metric did not include viewers who had watched for less than three seconds in the count.
Discounting the shorter views - including people who had ignored a video in their news feed - inflated the average viewing times for each video.
It was also criticised for counting a video as being viewed after three seconds.
Video cash
Now, a handful of small advertisers are suing Facebook for unfair business conduct and fraud.
As part of their case, they have viewed thousands of internal Facebook records and claim these show the company knew about the issue in 2015.
The plaintiffs claim that a Facebook engineering manager followed up on advertisers' complaints that dated back to early 2015, saying there had been "no progress on the task for a year".
Facebook developed a "no PR" strategy to avoid drawing attention to the error, according to the court filing.
Their case hinges on the fact that the numbers provided by Facebook meant advertisers put more money into its video ads than those on other platforms, reports BBC.
In response, Facebook said: "This lawsuit is without merit and we've filed a motion to dismiss these claims of fraud.
"Suggestions that we in any way tried to hide this issue from our partners are false."
Facebook now has a dedicated metrics team and allows third parties and experts to review its measurements regularly.
