Your Node.js app builds successfully but, when it's run, you get a "cannot find module" error like:
Error: Cannot find module 'foobar'
First, you should temporarily disable any module caching:
$ heroku config:set NODE_MODULES_CACHE=false
Next, you should ensure that you aren't accidentally checking your modules into git.
Finally, you should check to ensure that your dependencies are correctly listed in package.json.