In February 2018, The Washington Post launched new lists of best-selling books. In an attempt to combine online and in-store sales, some of the new lists incorporated data from both NPD BookScan and Amazon. For the first three months, all the lists were published correctly. But in late May 2018, the software that merges the two data sets began to experience substantial, intermittent errors, rendering the lists inaccurate. Two causes have been identified. One was related to how the software ingested and combined data from the two sources. The other started in late December 2018, when a change in one data source led the software to begin inadvertently removing the top bestseller, causing the books to be listed in the wrong order and ignoring some sales from top-selling titles. These errors were caught in June 2019. All errors have been corrected and the lists have been republished online and labeled with corrections. The Post is now hand-checking every list before publication to ensure accuracy.