I riepilogate with corrections and updates:

I've an Ajax Accordion that gets data from a database, in its content
there is a Repeater that gets data from another table, depending which
header is selected in the accordion.

in the Repeater I've a checkbox (one for eache repeated field) checked
by default that I select/deselect triggering the method
OnCheckedChanged which updates the record in the table connected to
the repeater.

this method OnCheckedChanged is triggered correctly (with the right
ClientID) when I deselect the a checkbox. I use the passed sender
checkbox's ClientID to identify and modify the record in the table

PROBLEM:
when I deselect the CheckBox, the correct sender ClientID is passed,
when I select the previously deselected checkbox a wrong ClientID is
passed!
in particolar, i found that it is passed the ClientID of last
unselected chechbox in the repeater

Please help, I'm going nuts!

here is the essential structure:

======================aspx
page========================================
<%@ Page Language="C#" AutoEventWireup="true"
CodeFile="Shipping.aspx.cs"
Inherits="test" MaintainScrollPositionOnPostback="true" %>

   <ajaxToolkit:ToolkitScriptManager EnablePartialRendering="true"
runat="Server" ID="ToolkitScriptManager1" />

     <ajaxToolkit:Accordion ID="MyAccordion" runat="server"
SelectedIndex="0" HeaderCssClass="accordionHeader"
           HeaderSelectedCssClass="accordionHeaderSelected"
ContentCssClass="accordionContent"
           FadeTransitions="false" FramesPerSecond="1"
TransitionDuration="1" AutoSize="None"
           RequireOpenedPane="true" SuppressHeaderPostbacks="false"
Width="900px" >

       <ContentTemplate>

           <asp:Repeater ID="Repeater1" runat="server"
DataSourceID="SqlDataSource1">
           <ItemTemplate>

                               <asp:CheckBox ID="ManifestCheckBox"
runat="server" AutoPostBack="true"
                                   Checked="True" OnCheckedChanged =
"ManifestCheckBox_CheckedChanged" />

           </ItemTemplate>
           </asp:Repeater>
       </ContentTemplate>
     </ajaxToolkit:Accordion>

   <asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:XXXXXXXXXXXXXXX %>"
       SelectCommand="SELECT * FROM [tblPICManifest] WHERE
([PICBatchID] =
@PICBatchID)">
       <SelectParameters>
           <asp:ControlParameter ControlID="selectedLbl"
DefaultValue=""
Name="PICBatchID"
               PropertyName="Text" Type="Int32" />
       </SelectParameters>
   </asp:SqlDataSource>

======================aspx.cs
code========================================

protected void Page_Load(object sender, EventArgs e)
   {

       if (!IsPostBack)
       else
       {
           ............
       }

protected void ManifestCheckBox_CheckedChanged(object sender,
EventArgs e)
{
       Control ctrSender = (Control)sender;
       string controlID = ctrSender.ClientID;

       ............

}

--
ciao
Vittorix