Rails: how to debug assets:precompile failure on cap deploy - ruby-on-rails

Question

I'm getting a cryptic error on cap deploy
$ cap deploy
. . .
* executing "cd -- /rails/myapp/releases/20130919002235 && RAILS_ENV=production RAILS_GROUPS=assets bundle exec rake assets:precompile"
servers: ["persephone"]
[persephone] executing command
. . .
** [out :: persephone] rake aborted!
** [out :: persephone] Caught Encoding::CompatibilityError at '["ok","(function() {': incompatible encoding regexp match (ASCII-8BIT regexp with UTF-8 string)
** [out :: persephone] (in /rails/myapp/releases/20130919002235/app/assets/javascripts/disk_files.js.coffee)
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/json-1.8.0/lib/json/pure/parser.rb:242:in `rescue in parse_string'
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/json-1.8.0/lib/json/pure/parser.rb:213:in `parse_string'
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/json-1.8.0/lib/json/pure/parser.rb:257:in `parse_value'
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/json-1.8.0/lib/json/pure/parser.rb:121:in `parse'
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/json-1.8.0/lib/json/common.rb:155:in `parse'
. . .
In trying to debug the problem I've tried completely removing disk_files.js.coffee and then the same error occurs for application.js.
I've then changed my application.js manifest so that it doesn't require anything and then I get a similar error for application.css
I've commented out my custom css and now I get:
** [out :: persephone] rake aborted!
** [out :: persephone] undefined method `[]' for nil:NilClass
** [out :: persephone] (in /rails/myapp/releases/20130919200133/app/assets/stylesheets/application.css)
** [out :: persephone] /rails/myapp/shared/bundle/ruby/2.0.0/gems/sprockets-2.10.0/lib/sprockets/sass_functions.rb:63:in `sprockets_context'
Each of these steps requires a git commit to origin and then a cap deploy.
FWIW- adding #encoding: UTF-8 as per SO#9857443 doesn't help.
Anyone know what's going on? How can I debug this?