Example you have button ABC and XYZ and you have Textbox QWE.
You need to specify ValidationGroup.
Let say:
<asp:textbox id="QWE" runat="server" validationgroup="Q"></asp:TextBox>
<asp:Button id="ABC" runat="server" validationgroup="Q"></asp:Button>
<asp:Button id="XYZ" runat="server"> </asp:Button>
or you can simply turnoff the validation of the XYZ button by setting the attribute to
CausesValidation='false'