Question : Before Update Event in Combo Box Cancel event

This is my code and attached are examples of errors I get.
Statement "Cancel = True" is causing the problem.

Error1 happens only in this example, but Error2 happens in all my combo boxes on various forms throughout the project where I have Cancel = True in the Before Update Event.

Private Sub Client_ID_BeforeUpdate(Cancel As Integer)

    If Nz(DLookup("[Client ID]", "[tblAttendance]", "[Session ID] = " & _
        Nz(Forms![frmSessionAttendance].Form.Recordset.[Session ID], 0) & _
        "AND [Client ID] = " & Nz(Me.[Client ID], 0)), 0) <> 0 Then
   
        MsgBox "This client already exists on this list."
        Me.Client_ID.Undo
        Cancel = True
    End If
End Sub

Answer : Before Update Event in Combo Box Cancel event

If you cancel the before update event of a control then you also need to cancel the form before update event and handle the form undo event. See  www.thenelson.name/#ReportFormTricks, "Check for required items" for details on how to do it.