Test.aspx
     <telerik:RadScriptManager ID="scriptManager" runat="server" />
            <telerik:RadComboBox ID="rcb" runat="server" Label="Medication:" AutoPostBack="true"
                Width="250" OnSelectedIndexChanged="rcb_SelectedIndexChanged">
                <Items>
                    <telerik:RadComboBoxItem Text="-- select --" Value="" />
                    <telerik:RadComboBoxItem Text="Benztropine 0.5 mg " />
                    <telerik:RadComboBoxItem Text="Buspirone 10 MG" />
                    <telerik:RadComboBoxItem Text="Haloperidol 100mg/ml" />
                    <telerik:RadComboBoxItem Text="Metaprolol 25 mg" />
                    <telerik:RadComboBoxItem Text="Quetiapin/Seroquel 400 mg" />
                    <telerik:RadComboBoxItem Text="Trazadone 150 mg" />
                    <telerik:RadComboBoxItem Text="Vitamin D2 50,000 units" />
                </Items>
            </telerik:RadComboBox>
            <label>SelectedIndex</label> <asp:Label ID="Label1" runat="server" Text=""></asp:Label>
Test.aspx.cs
protected void rcb_SelectedIndexChanged(object sender, RadComboBoxSelectedIndexChangedEventArgs e)
{
    Label1.Text = rcb.SelectedIndex.ToString();
    rcb.SelectedIndex = 0;
}
When "Benztropine 0.5 mg " is selected, the SelectedIndex in the selected index changed method is incorrectly set to 0, instead of 1. Selecting any other item in the list results in the correct SelectedIndex. I spent a lot of time debugging the code until I noticed that "Benztropine 0.5 mg " has a space at the end.

<telerik:RadStyleSheetManager runat="server" ID="RadStyleSheetManager">
    <StyleSheets>
            <telerik:StyleSheetReference Name="Telerik.Web.UI.Skins.BootstrapLite.ComboBOx.Bootstrap.css" Assembly="Telerik.Web.UI.Skins" />
    </StyleSheets>
</telerik:RadStyleSheetManager>

<script>
    function OnClientLoad(sender, args) {
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }

    function OnClientSelectedIndexChanged(sender, args) {
        $telerik.$(sender.get_dropDownElement()).find(".rcbSelected").removeClass("rcbSelected");
        var selectedItem = sender.get_selectedItem();
        if (selectedItem) {
            $telerik.$(selectedItem.get_element()).addClass("rcbSelected");
        }
    }
</script>

<style>
    div.RadComboBoxDropDown .rcbSelected {
        color: red;
        background-color: lightgreen;
    }
</style>
<telerik:RadComboBox ID="RadComboBox1" runat="server" RenderMode="Lightweight" OnClientSelectedIndexChanged="OnClientSelectedIndexChanged" OnClientLoad="OnClientLoad" MaxHeight="150">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" />
        <telerik:RadComboBoxItem Text="Item 2" />
        <telerik:RadComboBoxItem Text="Item 3" Selected="true" />
        <telerik:RadComboBoxItem Text="Item 4" />
    </Items>
</telerik:RadComboBox>