When you're fooling around with master pages, occasionally you'll run into a situation where you must change the master of a content page from one to another (masterA.master ---> masterB.master). The problem you'll sometimes encounter when doing this is that your <asp:Content> controls do not match up (they are named differently or there is a different number of them) between one master page file and another: your content pages will then be hosed with improperly assigned ContentPlaceHolder properties on the <asp:content> controls.
For example, say you have the following <asp:content> controls already on your page (because of the master page the content page is assigned to):
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder_Etc" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder_ActionsTitle" Runat="Server">
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder_ActionsSubTable" Runat="Server">
</asp:Content>
You then swap master pages and this master page has two more ContentPlaceHolders on it; and sometimes the IDE will do the following:
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder_Etc" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder_ActionsTitle" Runat="Server">
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder_ActionsSubTable" Runat="Server">
</asp:Content>
<asp:Content ID="Content4" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
</asp:Content>
<asp:Content ID="Content5" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">
</asp:Content>
Note how the final two are given generic ContentPlaceHolderX values to the ContentPlaceHolderID property. In my master page file, however, they are (specifically):
<asp:Content ID="Content4" ContentPlaceHolderID="ContentPlaceHolder_Graphic" Runat="Server">
</asp:Content>
<asp:Content ID="Content5" ContentPlaceHolderID="ContentPlaceHolder_TitleIntro" Runat="Server">
</asp:Content>
Be mindful that just because it "looks" right, it still may not function right. My recommendation? Go through and make the change globally and then do a global name replacement of ContentPlaceHolder1 ----> ContentPlaceHolder_Graphic, or whatever you choose.