For the time being you can use the following workaround:
ASPX:
		<div style="border: 1px solid red; height: 300px; width: 400px; overflow: scroll;">
			<div style="border: 1px solid green; height: 500px; width: 200px;">
				<telerik:RadDockZone ID="RadDockZone1" runat="server" RenderMode="Lightweight">
					<telerik:RadDock ID="RadDock1" runat="server" Resizable="true" RenderMode="Lightweight">
						<ContentTemplate>
							<div style="border: 1px solid blue; height: 100px;"></div>
						</ContentTemplate>
					</telerik:RadDock>
				</telerik:RadDockZone>
			</div>
		</div>
		<script>
			var Browser = Telerik.Web.Browser;
			$ = $telerik.$;
			Telerik.Web.UI.RadDock.prototype._initResizableWidget = function()
			{

				if(this._resizableWidget && this._resizableWidget.dispose)
					this._resizableWidget.dispose();

				if(!this._resizable) return;

				if(Browser.webkit) {
					//need to kickstart the css transform, because of a quirk in the layout engine of webkit/blink when it comes to position:static inside position:relative
					this._kickStartCssTransform();
				}

				this._resizableWidget = new Telerik.Web.UI.Widgets.Resizable(this._element, {
					handles: this._getAvailableHandles(), appendHandleToElement: true
				});

				if(this._renderingView)
					this._setResizeLimit();

				if(this._collapsed)
					this._resizableWidget.hideHandles();

				this._resizableWidget.add_resizeStart($.proxy(this._resizeStartHandler,this));
				this._resizableWidget.add_resizing($.proxy(this._resizingHandler, this));
				this._resizableWidget.add_resizeEnd($.proxy(this._resizeEndHandler, this));
			}
		</script>

As of Q1 2016 the elasticity of RadDock is not behaving properly due to  some fixed font styles like:
.RadDock .rdCommandButton
 {
     font-size: 16px;
 }

Possible workaround:
div.RadDock .rdCommandButton
 {
     font-size: 1.2em;
 }

The client state of the dock changes, e.g, 80% to 80px. The pixel value affects the dock after a post back.

Possible workaround:

<telerik:RadDock runat="server" ID="dock2" Width="80%">
    <ContentTemplate>
        <telerik:RadButton runat="server"></telerik:RadButton>
    </ContentTemplate>
</telerik:RadDock>

<script>
    Telerik.Web.UI.RadDock.prototype.saveClientState = function () {
        var DECIMAL_RADIX = 10;
        var dock = this;
        var width = dock.get_width();
        var height = dock.get_height();
        var top = dock.get_top() ? parseInt(dock.get_top(), DECIMAL_RADIX) : null;
        var left = dock.get_left() ? parseInt(dock.get_left(), DECIMAL_RADIX) : null;
        var expandedHeight = parseInt(dock._expandedHeight, DECIMAL_RADIX);

        var state = {
            Top: top,
            Left: left,
            DockZoneID: this.get_dockZoneID(),
            Collapsed: this.get_collapsed(),
            Pinned: this.get_pinned(),
            Resizable: this.get_resizable(),
            Closed: this.get_closed(),
            Width: width,
            Height: height,
            ExpandedHeight: expandedHeight,
            Index: this.get_index(),
            IsDragged: this.get_isDragged()
        };

        return Sys.Serialization.JavaScriptSerializer.serialize(state);
    };
</script>

When there is a RadDock with one or more disabled commands, you focus the dock and you use one of the default keyboard shortcuts for the disabled command(s), the keydown browser event will be disabled.

The default shortcuts are:

    ESC - Close command
    Ctrl + F11 - Toggle Expand / Collapse command
    Crtr + F12- Toggle Pin / UnPin command

When you dock a resizable RadDock (Resizable="true") in a RadDockZone, its drag handle is displaced in Chrome, which prevents you from resizing it when there are multiple docked docks.

When you place a resizable RadDock control (Resizable="true") in a RadDockZone without specified size and you close the dock, a JavaScript error is thrown.

The rounded corners of RadDock controls that are using the Silk or Glow skin cannot be disabled.

For the time being you can either declare some command or place the following JavaScript workaround below the RadDock's declaration:
		<telerik:RadDockZone ID="RadDockZone1" runat="server" Height="100%" MinWidth="50">
			<telerik:RadDock ID="RadDock1" runat="server" RenderMode="Lightweight" Title="title"
				DefaultCommands="None" DockHandle="TitleBar" DockMode="Docked">
				<ContentTemplate>
					<div>
						Content...
					</div>
				</ContentTemplate>
			</telerik:RadDock>
		</telerik:RadDockZone>
		<script>
			Telerik.Web.UI.Dock.LightweightView.prototype._sizeTitleElement = function () {
				var titleElem = this.get_titleElement(),
					titleBarElem = this.get_titleBar(),
					commandsElem = this.get_commandsContainer();
				if (commandsElem == undefined) {
					titleElem.style.width = Math.max(0, parseInt(titleBarElem.clientWidth)) + "px";
				}
				else {
					titleElem.style.width = Math.max(0, parseInt(titleBarElem.clientWidth) - parseInt(commandsElem.clientWidth)) + "px";
				}
				
			}
		</script>

When the browser or the page, on which a RadDock is placed, is configured with a culture that uses comma-separated values and you set the Width, Height, Top, Left properties with such a value, an exception will be thrown.

When the content container of RadDock is configured to be a drag handle via the following code

dock.set_handle(dock.get_contentContainer());

a JavaScript error will be thrown if you call the 'clone' client-side function of the control

dock.clone()

Setting the RenderMode=Auto to a Dock control results in a JS error on the client.

Expected is to choose the most appropriate render mode among the existing such.

The animation of the RadDock control is causes performance issues under IE8 and due to that the expanding/collapsing and the docking/undocking functionalities are not working as expected.

Disabling the animation resolves the issue: 
EnableAnimation="false"

The RadDock control cannot be expanded when it is set in lightweight render mode and its property Collapsed is set to true.

You can avoid this issue by setting the dock in collapsed state via its client-side API:

<script type="text/javascript">
    function OnClientInitialize(sender, args) {
        sender.set_collapsed(true);
    }
</script>
<telerik:RadDock runat="server" ID="RadDock1" OnClientInitialize="OnClientInitialize">
</telerik:RadDock>

The appearance of the RadDock's title bar is broken when the control is configured with any of the following built-in skins in a RTL layout.

It should not be possible to add docks in a RadDockZone, the property AllowedDocks of which does not include the IDs of the included docks.

The RadDock in a vertical RadDockZone is not expanded correctly the first time when it is collapsed by default and its Height is set. If Resizable is set to true, the dock will never expand.

When some content is entered in a RadDock, the project is closed, opened again and new elements are added in the content via the Visual Studio's designer, the elements that are included after the reloading of the project will disappear from the designer, after editing the dock.

The CSS class "RadDock_Default" will be applied to the placeholder of a RadDock, regardless of the built-in skin that is specified for the control.

When a RadDock without having its height property set is collapsed and then expanded, an explicit height is set to its wrapper as well as to its content wrapper, so that it cannot be resized. The workaround is to remove both heights on DockExpandCollapseCommand:

ASPX:

        <telerik:RadDock runat="server" ID="RadDock1" RenderMode="Classic" Resizable="true">
            <ContentTemplate>
                <asp:Button ID="btn" runat="server" Text="Click me" OnClientClick="toggleVisibility(); return false;" />
                <div id="divExpand" style="display: none; height: 400px;">
                    Expanded
                </div>
            </ContentTemplate>
            <Commands>
                <telerik:DockExpandCollapseCommand OnClientCommand="DockExpandCollapseCommand" />
            </Commands>
        </telerik:RadDock>

JavaScript:

        <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
            <script type="text/javascript">
                function toggleVisibility() {

                    var divExpand = $get('divExpand');

                    if (divExpand.style.display == 'none') {
                        divExpand.style.display = 'block';
                    }
                    else {
                        divExpand.style.display = 'none';
                    }
                }

                function DockExpandCollapseCommand(sender, args) {

                    if (!sender.get_collapsed()) {
                        sender.get_contentContainer().style.height = "";
                        sender.get_element().style.height = "";
                    }
                }
            </script>
        </telerik:RadCodeBlock>