I was writing a spec for a method that used a built-in Rails helper method, image_path. Now my method worked on my development environment, but did not work when my tests ran, and instead threw a:
Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError: path/to/asset.png isn't precompiled
It took me more than a few minutes to remember that I needed to change a setting in the test.rb envoronment file.
# change config.assets.compile = false # to config.assets.compile = true
Hope this helps someone else out.