Sometimes, the following error could show in the design view of the individual (aspx) page. That means something wrong or not suitable tags in the (Page.Master) file of your project.

The page has one or more <asp:Content> controls that do not correspond with <asp:ContentPlaceHolder> controls in the Master Page.
Whenever you face with this kind of error, please don’t forget to check out the following tips:
  • Remove Comments <%—-%>
  • Instead of <title />, <script />, <input /> replace with like <title><title/>, <script></script>
  • Make sure the ContentPlaceHolderID in 
    <asp:Content ID="contentMain" ContentPlaceHolderID="contentPlaceHolderMain" Runat="Server">
    matches id in each individual aspx page
    <asp:ContentPlaceHolder id="contentPlaceHolderMain" runat="server" />
Problem are solved when I got above problem.