I just did something silly while applying the January 2010 PSU to an 11gR1 ORACLE_HOME, and thought I'd share, in case someone else is Googling for the error message.
[oracle@dbserv patches]$ cd 9209238 [oracle@dbserv 9209238]$ opatch apply Invoking OPatch 11.1.0.6.2 Oracle Interim Patch Installer version 11.1.0.6.2 Copyright (c) 2007, Oracle Corporation. All rights reserved. (some OPatch output snipped) ApplySession failed: Patch ID is null. System intact, OPatch will not attempt to restore the system OPatch failed with error code 73
A closer look at the installation prereqs reveals the following statement: "You must use the OPatch utility version 11.1.0.6.7 or later," which I confess that I missed the first time around. D'oh. After installing the latest version of OPatch, the "opatch apply" command worked as expected.
Lessons reinforced:
- Even if you think you're up-to-date on prerequisites, triple-checking is a good idea
- Even humble tools like the OPatch utility can change pretty frequently
- Test systems are useful for testing your patching process, not just patches themselves
(Everyone join in, now: "Thanks, Mr. Obvious, you're a life saver!") 
