I seem to be asked this a lot so i thought i would mention this online so hopefully people will no longer ask me, they’ll find the answer when they ask google
You are trying to inherit from a masterpage and you recieve the following error
the page has one or more < asp:content> that do not correspond with < asp:ContentPlaceHolder> controls in masterpage
Script tags are the culprit around 80% of the time. For some reason the visual studio designer doesnt like not fully closing your script tags at design time.
Change all of you script tags from this
< script type="text/javascript" src="js/myjs.js" />
correcting it to this:
< script type="text/javascript" src="js/myjs.js" >< /script>
Any you should be good to go.