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In the SQL Command editor (click on a data adapter, in property window find
the SELECT command property, drill down to the CommandText sub-property and
click the button to edit) if you use a UNION clause in your SQL statement
the editor will complain that it cannot graphically support the UNION
statement. It seems to be referring to the top pane where it diagrams your
table relationships, because that pane becomes disabled. Nontheless, the
statement will execute correctly and return correct results (assuming you
format the SQL statement correctly, obviously).

Unfortunately, when you close that editor, it once again complains that it
cannot support UNION graphically in the editor (thanks for the reminder!)
but it will also fail to update the code behind. That's the bug.

I have only tested this using the objects from System.Data.OracleClient, but
I'm betting the bug is in the editor, not the specific client. I bet I will
be able to reproduce it using OleDB objects for example.