If you set a RowHeight value that is smaller than the height of a pinned row, the last row is clipped when you scroll to bottom. Based on the RowHeight and the number of pinned rows, you can have the last row partially clipped or fully clipped. Also, you can have more than one rows that are cut.

To work this around set the Padding and MinHeight properties of the GridViewPinButton.

<telerik:RadGridView.Resources>
       <!-- In case you are using NoXaml dlls, set the following property to the Style object: -->
       <!-- BasedOn="{StaticResource GridViewPinButtonStyle}" -->
	<Style TargetType="telerik:GridViewPinButton">
		<Setter Property="MinHeight" Value="0" />
		<Setter Property="Padding" Value="2" />
	</Style>
</telerik:RadGridView.Resources>