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In the "Validation" section of the "Fields" page, It seems that the error processing is only invoked -- and error messages displayed -- only when the "Edit" button is clicked at the bottom of the page to save the changes. Having it fire on exiting the field(s) would be yet better so users can see the message about problems in the context of actually entering the data and closer to real-time.

You could write jQuery code for your own Client-Side validation.