Resolved
This issue has been resolved.
Monitoring
The engineers triggered a reload of PgBouncer for customers of Heroku Postgres with Connection Pooling. Affected Heroku Postgres databases should no longer be reporting the errors.
Update
The engineers have deployed a fix.
Retroactive
Heroku Postgres with Connection Pooling enabled may show increased number of errors like:
PG::TRSerializationFailure: ERROR: could not serialize access due to concurrent update