Change effective on 28 October 2013
When deploying a Rails 3 app, if
rake assets:precompile is detected as a valid rake task, and no asset manifest file is present (indicating that the task should be skipped), the precompile command is now expected to succeed. If it does not succeed the deploy process will exit, and the results of the
assets:precompile task output to the screen for debugging.
If an asset pre-compilation fails on Heroku ensure that you can run it locally without error:
$ bundle exec rake assets:precompile RAILS_ENV=production RAILS_GROUPS=assets
See Troubleshooting Rails 3 asset precompilation for more information.