How to resolve 'ORA-02291: integrity constraint ... violated - parent key not found'?

This is a typical Oracle error and normally the problem can be resolved by adding a parent record as specified in the error message.  But, in this case, the parent record already exists; therefore, I believe, something is going on behind the scene and I have exhausted all my wits/resources and desperately need your help!

We are using Oracle 10.2g with Oracle form Builder 6i.  I keep receiving the following error message on the bottom (console) of the form while trying to insert a record to one of the blocks on a form:

An error prevented your changes from being saved.