The tabs of SlidingPanes are not rendered (sized) properly is the SlidingPane is containing a nested Splitter. The issue is not reproducible with 2014.2.724 version of the controls.

Reproduction code:
        <telerik:RadSplitter ID="RadSplitter2" runat="server" Skin="Metro">
            <telerik:RadPane ID="RadPane1" runat="server" Width="22px">
                <telerik:RadSlidingZone ID="RadSlidingZone1" runat="server" Width="22px">
                    <telerik:RadSlidingPane ID="RadSlidingPane2" Title="What If" runat="server">
                        <telerik:RadSplitter ID="WhatIfPaneSplitter" runat="server" Width="50px">
                            <telerik:RadPane ID="RadPane2" runat="server"></telerik:RadPane>
                            <telerik:RadPane ID="RadPane3" runat="server"></telerik:RadPane>
                        </telerik:RadSplitter>
                    </telerik:RadSlidingPane>
                </telerik:RadSlidingZone>
            </telerik:RadPane>
            <telerik:RadPane ID="RadPane4" runat="server"></telerik:RadPane>
        </telerik:RadSplitter>