Collection was modified; enumeration operation may not execute
<p>I keep getting the following warning (not error) in a production ASP.Net application:</p>

<pre>System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.Web.UI.ControlCollection.ControlCollectionEnumerator.MoveNext()
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)
at System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output)
at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer)
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at DMAC.UI.Master.Render(HtmlTextWriter writer)
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at System.Web.UI.Page.Render(HtmlTextWriter writer)
at Telerik.Web.UI.RadAjaxControl.RenderPageInAjaxMode(HtmlTextWriter writer, Control page)
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
at System.Web.UI.Page.Render(HtmlTextWriter writer)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
--- End of inner exception stack trace ---
at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.scan_history_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)</pre>

<p>Here is the code for the Render method where the error is occurring:</p>

<pre><code>protected override void Render(HtmlTextWriter writer)
    {
        Header header = new Header();
        List&lt;Menu&gt; menus = DMAC.Application.Current().Menus;
        Menu menu = menus.Find(m =&gt; m.MenuId == MenuId);

        SiteMapWindow siteWindow = new SiteMapWindow();
        siteWindow.OpenerId = Menu.PATHID;
        this.Page.Form.Controls.Add(siteWindow);

        SupportWindow supportWindow = new SupportWindow();
        supportWindow.OpenerId = Menu.SUPPORTID;
        this.Page.Form.Controls.Add(supportWindow);

        this.Page.Form.Controls.AddAt(0, menu);
        this.Page.Form.Controls.AddAt(0, header);

        Scripts.GoogleAnalytics.RegisterScript(this.Page);
        base.Render(writer);
    }
</code></pre>

<p>I cannot recreate the error myself.  Any ideas?</p>
