sharepoint online :error occurs when add a reference to the sp.runtime.js

Hi. I have a test version up of sharepoint online.
I have created a copy of a sitecollection for evaluation and understood that in some cases sp.js and sp.runtime.js does not load.
That site use JSOM so I added reference as the following post.
http://social.msdn.microsoft.com/Forums/sharepoint/en-US/ede0b05a-7365-481b-a7ce-ef1096fef983/sharepoint-2013-javascript-not-getting-loaded-some-times?forum=appsforsharepoint
But then error occured at sp.runtime.js, row 2 59,270th character, "SCRIPT5007: Unable to get property '/sites/root-eval/_api/contextinfo' of undefined or null reference".
If this is a common problem, please tell me how to respond.