two concurrent ANALYZE queries on the same table can cause this this error.
some access control/DDL statements (e.g. GRANT) run concurrently can cause this error.
Can a "VACUUM ANALYZE $TABLE" (e.g., from autovacuum) concurrent with a "UPDATE $TABLE" cause this error?
Can two concurrent "UPDATE $TABLE" queries which touch completely disjoint sets of rows (e.g. where clause on primary key with disjoint filters) cause this error?