My archive log destination directory was getting filled up and i got alert that thresholds had been reached. I didn't know why until i looked at the archive log script backup log.   This is what it showed in the log:

RMAN-06003: ORACLE error from target database:
ORA-04031: unable to allocate 41152 bytes of shared memory ("shared pool","DBMS_RCVMAN","PL/SQL MPCODE","BAMIMA: Bam Buffer")
ORA-06508: PL/SQL: could not find program unit being called: "SYS.DBMS_RCVMAN"

RMAN-06438: error executing package DBMS_RCVMAN in TARGET database
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-06429: TARGET database is not compatible with this version of RMAN

I tried to login to rman manually to see what was going on and got the same error:

# rman

Recovery Manager: Release 10.2.0.3.0 - Production on Thu Dec 17 00:36:09 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

RMAN> connect target /

ORACLE error from target database:
ORA-04031: unable to allocate 41152 bytes of shared memory ("shared pool","DBMS_RCVMAN","PL/SQL MPCODE","BAMIMA: Bam Buffer")
ORA-06508: PL/SQL: could not find program unit being called: "SYS.DBMS_RCVMAN"

error executing package DBMS_RCVMAN in TARGET database
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-06429: TARGET database is not compatible with this version of RMAN

RMAN> exit

I then logged into sqlplus to check the rman packages to see because there was no recent upgrades of any kind.

# sqlplus '/ as sysdba'

SQL*Plus: Release 10.2.0.3.0 - Production on Thu Dec 17 00:36:31 2009

Copyright (c) 1982, 2006, Oracle.  All Rights Reserved.

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options

SQL> SQL> select dbms_rcvman.getpackageversion from dual;
select dbms_rcvman.getpackageversion from dual
                                          *
ERROR at line 1:
ORA-04031: unable to allocate 41152 bytes of shared memory ("shared
pool","DBMS_RCVMAN","PL/SQL MPCODE","BAMIMA: Bam Buffer")

I tried flushing my shared_pool and then checked to see how large my shared_pool was:

SQL> alter system flush shared_pool;

System altered.

SQL> select dbms_rcvman.getpackageversion from dual;
select dbms_rcvman.getpackageversion from dual
                                          *
ERROR at line 1:
ORA-04031: unable to allocate 41152 bytes of shared memory ("shared
pool","DBMS_RCVMAN","PL/SQL MPCODE","BAMIMA: Bam Buffer")

SQL> show parameter shared_pool

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
shared_pool_reserved_size            big integer 10M
shared_pool_size                     big integer 200M

Shared_pool size was small .. so i increased it and that fixed the problem:

SQL> alter system set shared_pool_size=300m;

System altered.

SQL> select dbms_rcvman.getpackageversion from dual;

GETPACKAGEVERSION
--------------------------------------------------------------------------------
08.00.04.00

backup is now running ok.

Hope this helps.