Code that reproduces the issue:
<asp:TextBox ID="Textbox1" runat="server" ValidationGroup="vg1" />
<telerik:RadButton ID="RadButton1" runat="server" ValidationGroup="vg1" Text="click" />
<asp:RequiredFieldValidator ID="Requiredfieldvalidator1" ErrorMessage="text box is empty" ControlToValidate="Textbox1" ValidationGroup="vg1" runat="server" />
<asp:ValidationSummary ID="Validationsummary1" runat="server" ValidationGroup="vg1" ShowMessageBox="true" />