Hi there

I hope I have not confused anyone. The official line from Metalink is:

This message is written to the alert log when the database is waiting for pmon to clean up processes, but pmon is unable to
clean them. The client connections to the server are causing the shutdown immediate or normal to hang. The workaround in such a situation will be to kill such processes ( Refer Note:1039146.6).

If there were no client connection at the time of issuing this command, then please do the following.

Before issuing the shutdown command issue:

Alter session set events '10400 trace name context forever, level 1';
shutdown immediate

This will produce a few trace files in your "user_dump_dest" directory."user_dump_dest" is an "init.ora" parameter. The trace file will be a systemstate dump. Upload the trace files. From the trace files, it can be seen that what processes are still active.

The other workaround in this case will be to shutdown with:

shutdown abort
startup restrict
shutdown normal

Since it happened only once, I would suspect that there might be some long running transaction going on in any of the sessions that were present at the time of shutdown. This could have caused because the shutdown was waiting for this transaction to roll back completely. There are no known bugs on this. The other possibility of this could be that there are some blocking processes present in the system, which caused some of the other sessions to hang. The trace file generated by event 10400 will help to analyze such a scenario

I hope this info helps. Thanks for your input.