Issue is reproducible with the code below:

1) Run the code below.

2) Click on "Postback" button or select an item from the dropdown.

3) A sever-side error is thrown - "Collection was modified; enumeration operation may not execute."

ASPX:

    <form id="form1" runat="server">
        <telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
        <telerik:RadButton ID="RadButton1" runat="server" Text="Postback" />

        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All"></telerik:RadFormDecorator>

        <telerik:RadDropDownList ID="cbListeLigue" DataSourceID="DetailsViewDataSource" DefaultMessage="Choisir une ligue" runat="server" AutoPostBack="True" DataTextField="CustomerID" DataValueField="CustomerID" Text="Ligue"></telerik:RadDropDownList>

        <asp:SqlDataSource runat="server" ID="DetailsViewDataSource" ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
            ProviderName="System.Data.SqlClient" SelectCommand="Select [CustomerID], [CompanyName], [Address], [City], [PostalCode], [Country] From [Customers]"
            OldValuesParameterFormatString="original_{0}" ConflictDetection="CompareAllValues" />

        <asp:SqlDataSource runat="server" ID="FormViewDataSource" ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
            ProviderName="System.Data.SqlClient" SelectCommand="SELECT [OrderID], [CustomerID], [EmployeeID],[OrderDate] FROM [Orders] WHERE ([CustomerID] = @CustomerID)"
            OldValuesParameterFormatString="original_{0}" ConflictDetection="CompareAllValues">
            <SelectParameters>
                <asp:ControlParameter ControlID="cbListeLigue" Name="CustomerID" PropertyName="SelectedValue"
                    Type="String"></asp:ControlParameter>
            </SelectParameters>
        </asp:SqlDataSource>

        <telerik:RadGrid ID="rGrid_ListeSaison" runat="server"
            DataSourceID="FormViewDataSource" AllowFilteringByColumn="True">
            <MasterTableView DataKeyNames="CustomerID" AutoGenerateColumns="false"
                DataSourceID="FormViewDataSource">
                <Columns>
                    <telerik:GridDateTimeColumn DataField="OrderDate" UniqueName="OrderDate">
                    </telerik:GridDateTimeColumn>
                    <%--<telerik:GridBoundColumn DataField="CustomerID" UniqueName="CustomerID"></telerik:GridBoundColumn>--%>
                </Columns>
            </MasterTableView>
        </telerik:RadGrid>
    </form>

You can choose one of the following workarounds:

1) Set the ControlsToSkip="GridFormDetailsViews,LoginControls,ValidationSummary"  property of the decorator.

2) Set the AllowFilteringByColumn property of the grid to false.

3) Remove the GridDateTimeColumn column of the grid.

Workaround - set the ControlsToSkip="Select" property of the decorator:
ASPX:
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" ControlsToSkip="Select" />
		<asp:DropDownList ID="Dropdownlist1" runat="server">
			<asp:ListItem Text="text1" />
			<asp:ListItem Text="text2" />
		</asp:DropDownList>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<LabelsAppearance RotationAngle="33"></LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="Item 1" />
						<telerik:AxisItem LabelText="Item 2" />
						<telerik:AxisItem LabelText="Item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

Workaround - avoid scrollbar decoration:

<telerik:RadFormDecorator runat="server" ID="rfd1" DecoratedControls="All" Skin="Default" ControlsToSkip="Scrollbars" />

Reproduction Steps:

1) Open the code below in Chrome with Telerik UI 2015.2.729 version.

2) Scroll till you reach the second blue div with the button.

3) Click the button to make an AJAX request.

4) Actual - the page is scrolled to the end of the first inner div. Expected - the page should not be scrolled.

       <telerik:RadFormDecorator runat="server" ID="rfd1" DecoratedControls="All" Skin="Default" />
        <div style="width: 600px; height: 500px; border: 3px solid green; overflow: scroll;">
            <div style="height: 1000px; border: 3px solid red;"></div>
            <asp:UpdatePanel ID="Updatepanel1" runat="server">
                <ContentTemplate>
                    <div style="height: 300px; border: 3px solid blue;">
                        <telerik:RadButton ID="RadButton1" runat="server" Text="Click" />
                    </div>
                </ContentTemplate>
            </asp:UpdatePanel>
        </div>

The issue is reproducible when you add a CSS class, responsible for styling the scrollbars, too lately and can also be reproduced on a page that contains no Telerik UI for ASP.NET AJAX controls:

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style>
        .RadForm.rfdScrollBars.RadForm_WebBlue div::-webkit-scrollbar,
        .RadForm.rfdScrollBars.RadForm_WebBlue div::-webkit-scrollbar-thumb:vertical,
        .RadForm.rfdScrollBars.RadForm_WebBlue div::-webkit-scrollbar-thumb:horizontal {
            background-color: red !important; /* choose your color */
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
        <telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
        <script>
            setTimeout(function () {
                Sys.UI.DomElement.addCssClass(document.documentElement, "RadForm RadForm_WebBlue rfdScrollBars");
            }, 10);
        </script>
        <div id="testDiv" style="overflow: scroll; height: 200px; width: 300px">
            <table>
                <tr style="height: 500px;">
                    <td></td>
                </tr>
            </table>
        </div>
    </form>
</body>
</html>

If you remove the timeout, however, the issue will be gone:

        <script>
        //    setTimeout(function () {
                Sys.UI.DomElement.addCssClass(document.documentElement, "RadForm RadForm_WebBlue rfdScrollBars");
         //   }, 10);
        </script>

Possible workarounds for scenarios when the RadFormDecorator control is involved:

    1) Remove the scorllbars styling by setting the ControlsToSkip property to "Scrollbars".

<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" ControlsToSkip="Scrollbars" />

     2) Use the following JavaScript workaround:
	<form id="form1" runat="server">
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<telerik:RadSkinManager ID="RadSkinManager1" runat="server" Skin="Glow"></telerik:RadSkinManager>
		<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" ControlsToSkip="Scrollbars" />
		<script>
			//JavaScript workaround:
			Telerik.Web.UI.RadFormDecorator.removeGlobalClassesToRootElement = function (rootElement) {
				rootElement.className = rootElement.className.replace(/rfdRtl|RadForm_\w+|rfdButton|rfdZone|rfdLabel|rfdHeading|rfdTextbox|rfdTextarea|rfdFieldset|rfdRadio|rfdCheckbox|rfdGrids/g, "").trim();
			};
			Sys.UI.DomElement.addCssClass(document.documentElement, "RadForm RadForm_<%=RadSkinManager.GetCurrent(this.Page).Skin%> rfdScrollBars");
		</script>
		<div id="testDiv" style="overflow: scroll; height: 200px; width: 300px">
			<table>
				<tr style="height: 500px;">
					<td></td>
				</tr>
			</table>
		</div>
	</form>

For the time being the following CSS can be used:

    <style>
        .rfdSelectBox.rfdSelectBox_Default.rfdSelectBoxDropDown {
            z-index: 4007;
        }
    </style>

Where 'Default' is the name of the skin.

The disabled inputs should be darker so it is visually clear that you cannot enter text in them. This does not happen with some skins (e.g. Windows7) with the lightweight rendermode.

Tabbing to decorated select element by RadFormDecorator, focuses the original select element. For the time being the following workaround can be used:

JavaScript:

        <script>
            function pageLoad() {
                $ = $telerik.$;
                $.each($(".rfdRealInput"), function (index, value) { value.setAttribute("tabindex", "-1") });
            }
        </script>

ASPX:
        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" RenderMode="Lightweight" />
        <asp:TextBox ID="Textbox1" runat="server" />
        <asp:DropDownList ID="Dropdownlist1" runat="server">
            <asp:ListItem Text="text1" />
            <asp:ListItem Text="text2" />
        </asp:DropDownList>
        <asp:TextBox ID="Textbox2" runat="server" />
        <telerik:RadButton ID="RadButton1" runat="server" />

In Q3 2013 Firefox and Chrome freeze when a container with a select element is made visible with JavaScript and RadFormDecorator is present on the page.

The issue will be fixed for the upcoming Q3 2013 SP1. For the time being possible workarounds are:

    - Setting the ControlsToSkip property of the RadFormDecorator to "Select".
    - OR placing the provided JavaScript code from the attached archive below the declaration of the RadFormDecorator.

Sample scenarios that reproduce the issue are:

    - Expanding an inner item from a RadPanelBar. The RadPanelBar has multiple Items that contain select elements and a RadFormDecorator is present on the page.
    - JavaScript is used to set the display property of a container with a select element to "block" for example, multiple containers with select elements are present on the page and RadFormDecorator is used. For example:
JavaScript:

<script type="text/javascript">
    function pageLoad() {
        $get("div1").style.display = "block";
    }
</script>

ASPX:

<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
<div id="div1" style="display: none;">
    <select>
        <option>text1</option>
        <option>text2</option>
        <option>text3</option>
    </select>
</div>
<div id="div2" style="display: none;">
    <select>
        <option>text1</option>
        <option>text2</option>
        <option>text3</option>
    </select>
</div>

For the time being you can use the attached sprite.

For the time being the following CSS workaround can be used:

    <style>
        .rfdSelect.rfdSelect_Default {
            min-height: 1.417em;
        }
    </style>

Typing a letter key must move the focus to the next instance of a visible node whose title begins with that printable letter.

For the time being you can either skip the selects elements from decoration:

ASPX:

        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" ControlsToSkip="Select" />
        <div id="div1">
            <telerik:ReportViewer ID="ReportViewer1" runat="server" Height="492px" Width="927px"></telerik:ReportViewer>
        </div>


Or decorate only particular zone that do not include the ReportViewer:

ASPX:
        <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" DecorationZoneID="div2" />
        <div id="div1">
            <telerik:ReportViewer ID="ReportViewer1" runat="server" Height="492px" Width="927px"></telerik:ReportViewer>
        </div>
        <div id="div2">
            <input type="text" name="name" value=" " />
            <select>
                <option>option1</option>
                <option>option2</option>
                <option>option3</option>
            </select>
        </div>