testing with sqlsmith shows that the following assertion doesn't hold:
FailedAssertion("!(XLogCtl->Insert.exclusiveBackup)", File: "xlog.c", Line: 10200)
The triggering statements always contain a call to pg_start_backup withthe third argument 'true', i.e. it's trying to start an exlusive backup.
I didn't manage to put together a stand-alone testcase yet.