Just keep track of your archives...

A customer of mine choose to manually delete some archives before the backup with RMAN started. RMAN didn't like that. RMAN noticed that there were archives missing and refused to start the backup.

I found the solution in a undocumented command. Here is the guiding text and the command (just in case you need it):

Code:
It may sometimes be neccessary to re-sync RMAN with details of which
archive-log files are available to be backed up.  This is most likely 
if archive logs have been removed from the archive log directory 
outside the control of RMAN.  The RMAN command to do this is not 
listed in the manuals, and is as follows: 

 change archivelog all validate;

HTH