I Just found this on the list which answers my question:
   
  Oracle  tablespaces in hot-backup mode are constantly changing, but the way 
they > change is known to oracle, so the backup files are good for recovery.
 > If you follow the correct Oracle procedure for how files are saved,
 > there wouldn't be a problem.  If they're not in hot-backup mode and
 > you're just grabbing the files, then they might not be consistent for
 > recovery. 


mark wragge <mark_t_wragge AT YAHOO DOT IE> wrote:
  I am using Networker Oracle Module to backup a database. The database is 
therefore put into hot backup mode which i thought meant that there was nothing 
written to the database while the backup is taking place. Sometimes we receive 
a warning stating that the file changed during the backup but this file is 
actually the database. How can this file change during the backup if it is in 
hot backup mode. Do i need to be concerned with these warnings. 

The message is:
Beginning backup of tablespace WRK_DATA...
Backing up datafile /SRVR1_db04/dbf/WRK_DATA_01.dbf...
/SRVR1_db04/dbf/WRK_DATA_01.dbf
save: Warning - `/SRVR1_db04/dbf/WRK_DATA_01.dbf' changed during save

Thanks, Mark 

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