The page has one or more asp:Content controls that do not correspond with asp:ContentPlaceHolder controls in the Master Page.

I kept getting this error in the design view:

The page has one or more <asp:Content> controls that do not correspond with <asp:ContentPlaceHolder> controls in the Master Page.

A few Google searches, this is what I found to work for me:

Setting
<title></title>
instead of
<title />
solved the problem.

http://forums.asp.net/t/1302708.aspx