I couldn't find this listed anywhere, so in case anybody else is faced with the same problem -
If you get an "uncaught exception: Permission denied to get property HTMLDivElement.parentNode" error showing in your js debugger with the Calendar Script installed, find '.originalTarget' in the code and replace it with '.target'.
This error may manifest itself when attempting to execute javascript validation on fields completely unrelated to calendar fields. For me it showed in Firefox but not IE.
Thanks to Matt who explained this on his Javascript Toolbox site.