Software Installation encountered an unexpected error while reading from the MSI file \\server\Software\Firefox\Firefox-22.214.171.124-en-US.msi. The error was not serious enough to justify halting the operation. The following error was encountered: The operation completed successfully.
I received this error when trying to add this MSI file to an existing group policy used for software distribution.
Even though this warning event was logged 10 (!) times in the event log, the group policy object was modified correctly and the MSI package was successfully deployed to all affected hosts.
I believe that this warning message can be safely ignored.