This can occur when the content located in the object that you are trying to access is breaking the functionality of the WYSIWYG. The best solution is to remove the offending content so that you can continue editing with these steps:
  1. When you receive the script warning, select "Stop Script".
  2. Click the blue link in the upper right hand corner that says "Use Plain Text Editor".
  3. View the content of the object and check for any customized code that may be causing the issue and remove it.
Note: If you have implemented customized code that cannot be removed as it has bearing on the styling and rendering of the object, then it is best to continue working in the Plain Text Editor as the WYSIWYG is not designed to interact which sophisticated code of this nature.