On November 18th, a replication bug was found in Postgres that affected the most recent versions of every Postgres release. The corruption that this bug may introduce could go undetected, and it manifests itself as a follower potentially having an inconsistent view of the data. For example, data could be present in the primary and not on the follower, or data deleted or updated on the primary and not from the follower. The likelihood of triggering this bug is higher for write-heavy workloads, such as many OLTP applications seen at Heroku.
We always recommend placing applications in maintenance mode and scaling down workers when performing a follower based changeover, and following this...