When a Sprockets object compilation fails (due to a syntax error in CoffeeScript or SASS), it cannot recompile after the error is fixed because it says \"file is already req
rake tmp:cache:clear deletes the temporary cache directory.
In my case, the following command worked:
rake tmp:clear
It should be:
Rails.application.assets.cache.clear
but this method only in master branch and not even in beta.
Update: added in 4.0.0.beta5.