Hello Giuseppe Reale,
Generally the behavior of the example is expected. ValidationSummary control is a separate control, which is used only to display validation errors, but you have to clear them manually. You can do this inside radGridView.RowEditEnded event when e.EditAction == GridViewEditAction.Cancel.
Could you please provide me with a little bit more information about your scenario? I'll be glad to help.
the Telerik team
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