ORA-19607 XXXXX is an active control file

Getting an error running new RMAN backups against a standby database:

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of delete command on ORA_DISK_2 channel at 01/08/2012 09:00:14
ORA-19607: /u20d/oradata/standby/stbyitst/standby01.ctl is an active control file

Turns out, there is a Metalink article to solve this issue: 444269.1

They don't call it a bug, but, somehow, the standby controlfile gets registered as a controlfile copy in the RMAN Metadata. The solution is to "UNCATALOG" this controlfile copy:

RMAN> list copy of controlfile;

using target database control file instead of recovery catalog
List of Control File Copies
===========================

Key S Completion Time Ckp SCN Ckp Time
------- - --------------- ---------- ---------------
6 X 18-AUG-10 2315490331 18-AUG-10
Name: /u20d/oradata/standby/stbyitst/standby01.ctl
Tag: TAG20100818T094510


RMAN> report obsolete;

RMAN retention policy will be applied to the command
RMAN retention policy is set to redundancy 1
Report of obsolete backups and copies
Type Key Completion Time Filename/Handle
-------------------- ------ ------------------ --------------------
Control File Copy 6 18-AUG-10 /u20d/oradata/standby/stbyitst/standby01.ctl

RMAN> change copy of controlfile uncatalog;

uncataloged control file copy
control file copy file name=/u20d/oradata/standby/stbyitst/standby01.ctl RECID=6 STAMP=727350310
Uncataloged 1 objects


RMAN> report obsolete;

RMAN retention policy will be applied to the command
RMAN retention policy is set to redundancy 1
no obsolete backups found

RMAN> delete obsolete;

RMAN retention policy will be applied to the command
RMAN retention policy is set to redundancy 1
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=135 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=133 device type=DISK
no obsolete backups found

After following the above procedure, no more problem with backups!