Here is a simple page that recreates the issue:
<%
@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="TelerikTest1._Default" %>
<%
@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
<!
DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<
html xmlns="http://www.w3.org/1999/xhtml" >
<
head runat="server">
<title>Untitled Page</title>
</
head>
<
body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="ScriptManager1" runat="server" EnableTheming="True">
</telerik:RadScriptManager>
<div>
<telerik:RadPanelBar ID="RadPanelBar1" runat="server" Width="1000">
<Items>
<telerik:RadPanelItem runat="server" Text="Customer Data" Expanded="True">
<Items>
<telerik:RadPanelItem Value="CustomerData" runat="server">
<ItemTemplate>
<table>
<tr>
<td>
<telerik:RadTextBox ID="txtLastName" MaxLength="50" Width="350px" Label="Last Name" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
<td width="20px"> </td>
<td>
<telerik:RadTextBox ID="txtFirstName" MaxLength="50" Width="350px" Label="First Name" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
</tr>
<tr>
<td>
<telerik:RadTextBox ID="txtStreet" MaxLength="50" Width="350px" Label="Street Address" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
<td width="20px"> </td>
<td>
<telerik:RadTextBox ID="txtCity" MaxLength="50" Width="350px" Label="City" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
</tr>
<tr>
<td>
<telerik:RadComboBox ID="cmbState" Label="State" LabelCssClass="LabelCustomerDDL" runat="server" Height="200px" Width="250px"
DropDownWidth="298px" EmptyMessage="Choose a State or Province" HighlightTemplatedItems="true"
EnableLoadOnDemand="true" MarkFirstMatch="true">
<HeaderTemplate>
<table style="width: 275px" cellspacing="0" cellpadding="0">
<tr>
<td style="width: 177px;">
State / Province
</td>
<td style="width: 60px;">
State Code
</td>
<td style="width: 40px;">
Country
</td>
</tr>
</table>
</HeaderTemplate>
<ItemTemplate>
<table style="width: 275px" cellspacing="0" cellpadding="0">
<tr>
<td style="width: 177px;">
Texas
</td>
<td style="width: 60px;">
TX
</td>
<td style="width: 40px;">
USA
</td>
</tr>
</table>
</ItemTemplate>
</telerik:RadComboBox>
</td>
<td width="20px"> </td>
<td>
<telerik:RadTextBox ID="txtPostalCode" MaxLength="50" Width="350px" Label="Postal Code" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
</tr>
</table>
</ItemTemplate>
</telerik:RadPanelItem>
</Items>
</telerik:RadPanelItem>
<telerik:RadPanelItem runat="server" Text="Vehicle Data">
<Items>
<telerik:RadPanelItem Value="VehicleData" runat="server">
<ItemTemplate>
<table>
<tr>
<td colspan="3">
<telerik:RadTextBox ID="txtVin" MaxLength="50" Width="350px" Label="VIN Number" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
</tr>
<tr>
<td>
<telerik:RadTextBox ID="txtLicense" MaxLength="4" Width="350px" Label="License Plate" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
<td width="20px"> </td>
<td>
<telerik:RadTextBox ID="txtYear" MaxLength="50" Width="350px" Label="Year" LabelCssClass="LabelCustomer" runat="server">
</telerik:RadTextBox>
</td>
</tr>
<tr>
<td>
<telerik:RadComboBox ID="cmbVehicleMake" Label="Make" LabelCssClass="LabelCustomerDDL" runat="server" Height="200px" Width="200px"
DropDownWidth="200px" EmptyMessage="Choose a Vehicle Make" HighlightTemplatedItems="true"
EnableLoadOnDemand="true" MarkFirstMatch="true">
</telerik:RadComboBox>
</td>
<td width="20px"> </td>
<td>
<telerik:RadComboBox ID="cmbVehicleModel" Label="Model" LabelCssClass="LabelCustomerDDL" runat="server" Height="200px" Width="200px"
DropDownWidth="200px" EmptyMessage="Choose a Vehicle Model" HighlightTemplatedItems="true"
EnableLoadOnDemand="true" MarkFirstMatch="true">
</telerik:RadComboBox>
</td>
</tr>
</table>
</ItemTemplate>
</telerik:RadPanelItem>
</Items>
</telerik:RadPanelItem>
</Items>
</telerik:RadPanelBar>
<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server">
</telerik:RadAjaxManager>
</div>
</form>
</
body>
</
html>