RMAN-06059: expected archived log not found

This can happen for a variety of reasons; the file has been manually moved or deleted, the archive log destination has recently been changed, the file has been compressed, etc.

Starting backup
current log archived
released channel: DISK01
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 14:24:55
RMAN-06059: expected archived log not found, lost of archived log
compromises recoverability
ORA-19625: error identifying file /u01/app/oracle/flash_recovery_area/TESTDB/2011_05_03/o1_mf_1_26vz057td.arc
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3 

Your options are either to restore the missing file(s), or to perform a crosscheck. To perform a crosscheck, run the following command from within RMAN:
change archivelog all crosscheck;
It is advisable to perform a full backup of the database at this point.

When an archive log crosscheck is performed, RMAN checks each archive log in turn to make sure that it exists on disk (or tape). Those that are missing are marked as unavailable. If you have got missing logs, this won't bring them back. It will allow you to get past this error and back-up the database though.