It is possible to see this error when running up the Platform Emulator. This is normally associated with browser caching.
The issue can show itself sometimes through a pop-up window with the error message Error parsing JSON response: 'Unexpected token <' . Logged in?
This somewhat odd error comes from the Salesforce Visualforce Remoting library.
To solve the issue go to your browser settings and clear cookies.
We have a video here that covers how to do this along with an explanation on why the cache can cause these issues.
Customer support service by UserEcho