Why are my builds failing with a node-sass error?

Resolution

The rest of the error message in the build log may contain the following:

Found bindings for the following environments:
- OS X 64-bit with Node.js 4.x

This usually happens because your environment has changed since running `npm
install`.
Run `npm rebuild node-sass` to build the binding for your current
environment.

This can occur when you have compiled modules locally and checked the resulting node_module folder into git before deploying. To fix this, we recommend removing the node_modules folder from your git repo and redeploying. You can find steps for this here https://devcenter.heroku.com/articles/troubleshooting-node-deploys#don-t-check-in-generated-directories