If this is PROVEN to have happened directly/shortly after user input, check if somehow biz logic blocks deletion of previously selected item or tries to set additional item automatically. If uncertain, prove first that you did not somehow try to set selected=true to more than 1 item via SQL or biz code

>I have never had this one on the Web site. I got two errors like this morning:
>"Cannot have multiple items selected in a DropDownList."
>
>The stack trace gives this:
>
>at System.Web.UI.WebControls.DropDownList.VerifyMultiSelect()
> at System.Web.UI.WebControls.ListControl.RenderContents(HtmlTextWriter writer)
> at System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
> at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer)
> at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)
> at System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output)
> at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
> at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
> at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
> at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
> at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
> at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
> at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer)
> at Framework.WebForm.Render(HtmlTextWriter toHtmlTextWriter)
> at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter)
> at System.Web.UI.Control.RenderControl(HtmlTextWriter writer)
> at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
>
>It is unclear how a user was able to make two selections in a select HTML control, which comes from DropDownList ASP.NET control.
>
>Anyone would have an idea what could be the trigger here?