File encoding of your package.json file
If you are using NPM or Grunt on Windows and are having issues running your tasks it would pay to check the file encoding on your package.json file. I was receiving the following error while trying to run Grunt:
"Warning: C:\dev\project\package.json : Unexpected token ? Use --force to continue."
The issue happened to be that VisualStudio had sneakily saved the file as UTF8-BOM, changing the encoding to UTF8 without the BOM solved this. I found the NPM bug which has been closed without a fix. Hopefully this will allow you to solve this quicker than I did!