We have multiple issues with the Rails 3 assets pipeline, and precompile errors are difficult to track down. We can switch to the Rails 4 asset pipeline now, as we are soon migrating to Rails 4 anyway.
Background info: https://discussion.heroku.com/t/using-the-rails-4-asset-pipeline-in-rails-3-apps-for-faster-deploys/205
removed unused image assets #172.
Move to Rails 4 assets pipeline #172
don't use sprockets_better_errors #172
put spinner.gif into asset pipeline #172