It's actually a bug in the Oracle 10.5.0.2 and 11.2.0.1.
Bug can be reproduced as following:
set NLS_TIMESTAMP_FORMAT in session.
Run any implicit or explicit TO_DATE conversion with unicode data.
Next implicit or explicit TO_TIMESTAMP with unicode data will trigger internal reset of timestamp format.
All consecutive TO_TIMESTAMP will fail and TO_CHAR of timestamp will produce invalid output.
Here is the code to test behaviour:
ALTER SESSION SET NLS_TERRITORY = 'AMERICA';
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS' NLS_TIMESTAMP_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF';
REM --- WORKS:
SELECT TO_TIMESTAMP('2013-06-24 18:15:10.312') FROM DUAL;
REM --- WORKS:
SELECT TO_DATE('2013-06-24 18:15:10') FROM DUAL;
REM --- WORKS:
SELECT TO_TIMESTAMP('2013-06-24 18:15:10.312') FROM DUAL;
REM --- WORKS:
SELECT TO_TIMESTAMP(x) FROM (SELECT CAST('2013-06-24 18:15:10.312' AS NVARCHAR2(30)) AS X FROM DUAL);
REM --- WORKS:
SELECT TO_DATE(x) FROM (SELECT CAST('2013-06-24 18:15:10' AS NVARCHAR2(30)) AS X FROM DUAL);
REM --- WORKS:
SELECT TO_TIMESTAMP('2013-06-24 18:15:10.312') FROM DUAL;
REM !!! FAILS!
SELECT TO_TIMESTAMP(x) FROM (SELECT CAST('2013-06-24 18:15:10.312' AS NVARCHAR2(30)) AS X FROM DUAL);
REM !!! FAILS!
SELECT TO_TIMESTAMP('2013-06-24 18:15:10.312') FROM DUAL;
REM --- WORKS:
SELECT TO_DATE('2013-06-24 18:15:10') FROM DUAL;