SELECT  to_date('14-JUN-2005','DD-MON-YYYY') FROM DUAL;

SELECT  to_date('14-JUN-2005','DD-MON-YYYY') FROM DUAL
                *
ERROR at line 1:
ORA-01843: not a valid month

CAUSE:
SELECT * FROM NLS_SESSION_PARAMETERS;

PARAMETER                                                VALUE
NLS_LANGUAGE                                        SIMPLIFIED CHINESE
NLS_TERRITORY                                           CHINA
NLS_CURRENCY                                         ￥
NLS_ISO_CURRENCY                                 CHINA
NLS_NUMERIC_CHARACTERS                   .,
NLS_CALENDAR                                         GREGORIAN
NLS_DATE_FORMAT                                   DD-MON-RR
NLS_DATE_LANGUAGE                             SIMPLIFIED CHINESE
NLS_SORT                                                    BINARY
NLS_TIME_FORMAT                                    HH.MI.SSXFF AM
NLS_TIMESTAMP_FORMAT                        DD-MON-RR HH.MI.SSXFF AM
NLS_TIME_TZ_FORMAT                              HH.MI.SSXFF AM TZR
NLS_TIMESTAMP_TZ_FORMAT                  DD-MON-RR HH.MI.SSXFF AM TZR
NLS_DUAL_CURRENCY                           ￥
NLS_COMP                                               BINARY
NLS_LENGTH_SEMANTICS                       BYTE
NLS_NCHAR_CONV_EXCP                    FALSE

17 rows selected.

SOLUTIONS:
SQL> alter session set nls_date_language=AMERICAN;
SQL> alter session set nls_date_format='DD-MON-YYYY';