One that we've added to our daily 'health' check is query the actlog for:

ANR8443E Command: Volume volume name in library library name cannot be
assigned a status of SCRATCH.

messages. We had some tapes brought back that were not scratch and inserted
into the 3494 library. The library put them away, but TSM didn't check them
in. They were still valid offsite tapes that TSM still thought were offsite.
We went on a disaster recovery test and didn't have a tape volume we needed
for a restore. Found out it was in the library in limbo. For a SCSI library
they would still be in the I/O port (if you have one).

Once bitten...

Bill Boyer
DSS, Inc.