Tim Peterson:

To paraphrase J. Cole, Facebook messes up on its math one time; shame on Facebook. Facebook messes up on its math a second time; shame on those of us blindly trusting Facebook’s math. Facebook messes up on its math a third time; time to make a list.
 
 To help myself and anyone else keep track of Facebook’s measurement errors, here’s an itemized list of each error Facebook has announced to date, a (hopefully) normal-speak explanation of what the error was, whether Facebook has corrected it or plans to, how it has/will correct the error and what impact the error had on the measurements Facebook reported.
 
 Average watch time of Facebook page videos