Opatch Fails with Patch ID is null and Error Code 73

This is an issue, I have come across in the past.  This issue can impact most Oracle software homes when applying patches if the incorrect OPatch version exists.

A. Steps to Reproduce Issue

The following command was executed when applying a patch:
oracle@lnx01:[AGENT]$ cd /u01/app/oracle/software/10170020
oracle@lnx01:[AGENT]$ $ORACLE_HOME/OPatch/opatch apply

B. Error Reported

The following output error was reported below:
Invoking OPatch 10.2.0.4.5
Oracle Interim Patch Installer version 10.2.0.4.5
Copyright (c) 2008, Oracle Corporation.  All rights reserved.

Oracle Home       : /u01/app/oracle/agent10g
Central Inventory : /u01/app/oracle/oraInventory
   from           : /etc/oracle/oraInst.loc
OPatch version    : 10.2.0.4.5
OUI version       : 10.2.0.5.0
OUI location      : /u01/app/oracle/agent10g/oui
Log file location : /u01/app/oracle/agent10g/cfgtoollogs/opatch/opatch2011-03-15_07-55-15AM.log
Patch history file: /u01/app/oracle/agent10g/cfgtoollogs/opatch/opatch_history.txt
ApplySession failed: Patch ID is null.
System intact, OPatch will not attempt to restore the system
OPatch failed with error code 73

C. Solution 

1. Downloading patch 6880880 for the version of the software release (ie. 10.2 or 11.1, 11.2 etc…) from My Oracle Support.

The patch will be in the following format:
p6880880_<release>_<os_platform>.zip

For example in 10gR2 (10.2), the OPatch will be:
p6880880_102000_SOLARIS64.zip

2. Verify current OPatch version:
oracle@lnx01:[AGENT]$ $ORACLE_HOME/OPatch/opatch version
3. Backup the old OPatch
oracle@lnx01:[AGENT]$ cd $ORACLE_HOME
oracle@lnx01:[AGENT]$ mv OPatch/ OPatch.orig
4. Update the latest patch
oracle@lnx01:[AGENT]$ cd /u01/app/oracle/software
oracle@lnx01:[AGENT]$ unzip -d $ORACLE_HOME p6880880_102000_SOLARIS64.zip
5. Verify OPatch version:
oracle@lnx01:[AGENT]$ $ORACLE_HOME/OPatch/opatch version

6. And then re-apply patch.