that's probably simply something called "an application bug"
It would be nice if you would tell us how you entered this date, because I'm sure that you didnt do it via sqlplus or anything like that.
Did you by any chance enter that value via OCI?
Did you by any chance use the OCI external datatype DATE?
If so, you should carefully read your Oracle Documentation and you would see that:
"When you enter a date in binary format by using the external datatype DATE, the database does not perform consistency or range checking. All data in this format must be validated before input."
For more details have a look at:
http://otn.oracle.com/docs/products/...yp.htm#1003747
HTH
Mike