System.Web.HttpUnhandledException

[Didi]

telerik ver = 2011.2.915.40 runtime = v4.0.30319

My user got this error "Exception of type 'System.Web.HttpUnhandledException' was thrown."
and this all the error

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.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.task_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

ToString:
System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
at System.Collections.CollectionBase.System.Collections.IList.get_Item(Int32 index)
at Telerik.Web.UI.GridGroupByExpressionCollection.get_Item(Int32 index)
at Telerik.Web.UI.GridGroupPanel.Ungroup(String index)
at Telerik.Web.UI.RadGrid.RaisePostBackEvent(String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
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.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.task_aspx.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

and here my grid on the page :

<telerik:RadGrid ID="RadGridTaskPanel" runat="server" GridLines="None"
            AllowPaging="true" PageSize="50" AllowSorting="True" AutoGenerateColumns="false"
            ShowGroupPanel="false" Width="100%" OnNeedDataSource="RadGridTaskPanel_NeedDataSource"
            OnItemDataBound="RadGridTaskPanel_ItemDataBound" OnSortCommand="RadGridTaskPanel_SortCommand"
            OnGroupsChanging="RadGridTaskPanel_GroupsChanging">
            <PagerStyle Position="TopAndBottom" Mode="NumericPages" />
            <MasterTableView CellSpacing="-1">
                <EditFormSettings>
                    <PopUpSettings ScrollBars="None" />
                </EditFormSettings>
                <ExpandCollapseColumn Resizable="False" Visible="False">
                    <HeaderStyle Width="20px" />
                </ExpandCollapseColumn>
                <RowIndicatorColumn Visible="False">
                    <HeaderStyle Width="20px" />
                </RowIndicatorColumn>
                <Columns>
                    <telerik:GridHyperLinkColumn DataTextFormatString="Details" UniqueName="ActionSelect"
                        DataTextField="FKID" DataNavigateUrlFields="URL,CurrentStep_DocApprovalStatusId"
                        DataNavigateUrlFormatString="{0}{1}" />
                    <telerik:GridBoundColumn DataField="FKId" />
                    <telerik:GridBoundColumn DataField="DaysAway" />
                    <telerik:GridBoundColumn DataField="DocType" />
                    <telerik:GridTemplateColumn HeaderText="Description" GroupByExpression="ShortDescription Group By ShortDescription" SortExpression="ShortDescription" UniqueName="DescriptionMerge">
                    <ItemTemplate>
                    </ItemTemplate>
                    </telerik:GridTemplateColumn>
                    <telerik:GridBoundColumn DataField="ShortDescription" />
                    <telerik:GridBoundColumn DataField="LongDescription" />
                    <telerik:GridBoundColumn DataField="CustomField1" />
                    <telerik:GridBoundColumn DataField="CustomField2" />
                    <telerik:GridBoundColumn DataField="Requestor_DisplayName" />
                    <telerik:GridBoundColumn DataField="Assigned_DisplayName" />
                    <telerik:GridBoundColumn DataField="ActionByDate" DataFormatString="{0:dd/MM/yyyy}" />
                    <telerik:GridBoundColumn DataField="RequestDate"  />
                    <telerik:GridBoundColumn DataField="CurrentStep" />
                    <telerik:GridBoundColumn DataField="CurrentStep_DocApprovalStatusId" />
                    <telerik:GridBoundColumn DataField="URL" />
                </Columns>
                <PagerStyle AlwaysVisible="True" Position="Top" />
            </MasterTableView>
            <ClientSettings AllowDragToGroup="True">
            </ClientSettings>
        </telerik:RadGrid>

the problem are..i can't replicate it again,any help?