ORA 01012 WHEN execute SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL;

 I encounter ORA 01012 WHEN execute SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL; in my ProC program. the whole scenarios are:
1. My platform is Solaris 10, Oracle 10.2g / 64bit
2. "alter system archive log current" failure, Oracle complaint about flash_recovery_area full, so I run
delete obsolete;
crosscheck backup;
delete expired backup;
3. Later I issued command "shutdown", oralce had no reponse after long time, so I issued command "shutdown abort" to shutdown the database, and then "startup" the database successfully to "READ WRITE",  user application can also accessed the database.
4. One of my ProC program will get ORA-01012 when execute SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL;