Applications now "pinned" to official buildpacks on first deploy
Change effective on 29 October 2015
Applications that do not explicitly have a buildpack configured will now automatically get “pinned” to the detected default buildpack after a successful build as if a user had run heroku buildpacks:set directly after the deploy.
This will prevent inadvertent changes of the detected buildpack, e.g. when adding a package.json (which would trigger detection as a Node.js application) to an existing Java/Python/PHP/… codebase, or if the detection order of the default buildpacks on Heroku changes for any reason in the future.