Cuando la aplicación se implementa con capistrano, se producen errores en production.log.

Compiled signup.css  (30ms)  (pid 31797)
Warning. Error encountered while saving cache /home/deployer/apps/example.com/releases/20140315211501/tmp/cache/sass/c76a96d592cb37dc7092a4e2f10ad8d3d22bcc8b/user_sessions.css.scssc: can't dump anonymous class #<Class:0x0000000356d120>

Compiled user_sessions/user_sessions.css  (16ms)  (pid 31797)
Compiled user_sessions.css  (20ms)  (pid 31797)
Warning. Error encountered while saving cache /home/deployer/apps/example.com/releases/20140315211501/tmp/cache/sass/a59f84087c66a48b1521281da138ce9e3ed4c217/welcome.css.scssc: can't dump anonymous class #<Class:0x0000000356d120>

Este es Gemfile.lock que estoy usando.

GIT
  remote: git://github.com/tomazzlender/letter_opener.git
  revision: 29b17fefe87b8fdc7ead987dbfabe355b59ff684
  specs:
    letter_opener (0.0.2)
      launchy

GIT
  remote: git://github.com/tomazzlender/premailer.git
  revision: 88af5e6f60ac78ef1d70d6f0a6f4b7612d2c5f7a
  specs:
    premailer (1.7.3)
      css_parser (>= 1.1.9)
      htmlentities (>= 4.0.0)

GIT
  remote: git://github.com/tomazzlender/snapshot.git
  revision: 0f0f3fd024bf5a3a846e999b845803ae61d870c0
  specs:
    snapshot (1.1.0)

GIT
  remote: https://github.com/37signals/mail_view.git
  revision: c8cc1315026ce85973ef56e628ac2e38a7a95563
  specs:
    mail_view (1.0.1)
      tilt

GIT
  remote: https://github.com/thoughtbot/capybara-webkit.git
  revision: 11c596f7a06dbda123c077440f9275905f89e869
  specs:
    capybara-webkit (0.13.0)
      capybara (>= 2.0.1)
      json

GEM
  remote: https://rubygems.org/
  specs:
    actionmailer (3.2.17)
      actionpack (= 3.2.17)
      mail (~> 2.5.4)
    actionpack (3.2.17)
      activemodel (= 3.2.17)
      activesupport (= 3.2.17)
      builder (~> 3.0.0)
      erubis (~> 2.7.0)
      journey (~> 1.0.4)
      rack (~> 1.4.5)
      rack-cache (~> 1.2)
      rack-test (~> 0.6.1)
      sprockets (~> 2.2.1)
    active_attr (0.7.0)
      activemodel (>= 3.0.2, < 4.1)
      activesupport (>= 3.0.2, < 4.1)
    activemodel (3.2.17)
      activesupport (= 3.2.17)
      builder (~> 3.0.0)
    activerecord (3.2.17)
      activemodel (= 3.2.17)
      activesupport (= 3.2.17)
      arel (~> 3.0.2)
      tzinfo (~> 0.3.29)
    activeresource (3.2.17)
      activemodel (= 3.2.17)
      activesupport (= 3.2.17)
    activesupport (3.2.17)
      i18n (~> 0.6, >= 0.6.4)
      multi_json (~> 1.0)
    addressable (2.2.7)
    ansi (1.4.3)
    arel (3.0.3)
    authlogic (3.1.0)
      activerecord (>= 3.0.7)
      activerecord (>= 3.0.7)
    builder (3.0.4)
    capistrano (2.9.0)
      highline
      net-scp (>= 1.0.0)
      net-sftp (>= 2.0.0)
      net-ssh (>= 2.0.14)
      net-ssh-gateway (>= 1.1.0)
    capybara (2.0.2)
      mime-types (>= 1.16)
      nokogiri (>= 1.3.3)
      rack (>= 1.0.0)
      rack-test (>= 0.5.4)
      selenium-webdriver (~> 2.0)
      xpath (~> 1.0.0)
    childprocess (0.3.6)
      ffi (~> 1.0, >= 1.0.6)
    chronic (0.6.7)
    coffee-rails (3.2.2)
      coffee-script (>= 2.2.0)
      railties (~> 3.2.0)
    coffee-script (2.2.0)
      coffee-script-source
      execjs
    coffee-script-source (1.2.0)
    css_parser (1.2.6)
      addressable
      rdoc
    erubis (2.7.0)
    eventmachine (1.0.0)
    exception_notification (3.0.0)
      actionmailer (>= 3.0.4)
      tinder (~> 1.8)
    execjs (1.3.0)
      multi_json (~> 1.0)
    faraday (0.8.4)
      multipart-post (~> 1.1)
    faraday_middleware (0.9.0)
      faraday (>= 0.7.4, < 0.9)
    ffi (1.2.0)
    hashie (1.2.0)
    highline (1.6.11)
    hike (1.2.3)
    htmlentities (4.3.1)
    http_parser.rb (0.5.3)
    i18n (0.6.9)
    journey (1.0.4)
    jquery-rails (2.0.3)
      railties (>= 3.1.0, < 5.0)
      thor (~> 0.14)
    json (1.8.1)
    kgio (2.7.2)
    launchy (2.1.0)
      addressable (~> 2.2.6)
    libwebsocket (0.1.7.1)
      addressable
      websocket
    mail (2.5.4)
      mime-types (~> 1.16)
      treetop (~> 1.4.8)
    marginalia (1.1.0)
      actionpack (>= 2.3, < 3.3)
      activerecord (>= 2.3, < 3.3)
    meta_request (0.2.1)
      rack-contrib
      rails
    mime-types (1.25.1)
    minitest (4.3.3)
    minitest-capybara (0.1.0)
      capybara (>= 1.0)
      minitest-matchers (>= 1.2)
    minitest-matchers (1.2.0)
      minitest (>= 2.5.0)
    minitest-rails (0.3)
      minitest (~> 4.0)
      rails (~> 3.0)
    minitest-rails-capybara (0.1)
      minitest-capybara (~> 0.1)
      minitest-rails (~> 0.1)
    multi_json (1.9.0)
    multipart-post (1.1.5)
    net-scp (1.0.4)
      net-ssh (>= 1.99.1)
    net-sftp (2.0.5)
      net-ssh (>= 2.0.9)
    net-ssh (2.3.0)
    net-ssh-gateway (1.1.0)
      net-ssh (>= 1.99.1)
    nokogiri (1.5.0)
    pg (0.17.1)
    polyglot (0.3.4)
    premailer-rails3 (1.3.1)
      premailer (~> 1.7)
      rails (~> 3)
    psych (1.3.4)
    quiet_assets (1.0.1)
      railties (~> 3.1)
    rack (1.4.5)
    rack-cache (1.2)
      rack (>= 0.4)
    rack-contrib (1.1.0)
      rack (>= 0.9.1)
    rack-ssl (1.3.3)
      rack
    rack-test (0.6.2)
      rack (>= 1.0)
    rails (3.2.17)
      actionmailer (= 3.2.17)
      actionpack (= 3.2.17)
      activerecord (= 3.2.17)
      activeresource (= 3.2.17)
      activesupport (= 3.2.17)
      bundler (~> 1.0)
      railties (= 3.2.17)
    railties (3.2.17)
      actionpack (= 3.2.17)
      activesupport (= 3.2.17)
      rack-ssl (~> 1.3.2)
      rake (>= 0.8.7)
      rdoc (~> 3.4)
      thor (>= 0.14.6, < 2.0)
    raindrops (0.8.0)
    rake (10.1.1)
    rdoc (3.12.2)
      json (~> 1.4)
    ruby-ole (1.2.11.3)
    rubyzip (0.9.6.1)
    sass (3.2.6)
    sass-rails (3.2.6)
      railties (~> 3.2.0)
      sass (>= 3.1.10)
      tilt (~> 1.3)
    selenium-webdriver (2.27.2)
      childprocess (>= 0.2.5)
      libwebsocket (~> 0.1.3)
      multi_json (~> 1.0)
      rubyzip
    sheets (1.1.0)
      nokogiri (>= 1.4.3.1)
      rubyzip (>= 0.9.4)
      spreadsheet (>= 0.6.5.2)
    simple_oauth (0.1.9)
    spreadsheet (0.6.8)
      ruby-ole (>= 1.0)
    sprockets (2.2.2)
      hike (~> 1.2)
      multi_json (~> 1.0)
      rack (~> 1.0)
      tilt (~> 1.1, != 1.3.0)
    thor (0.18.1)
    tilt (1.4.1)
    tinder (1.9.1)
      eventmachine (>= 0.12.0, < 2)
      faraday (~> 0.8)
      faraday_middleware (~> 0.8)
      hashie (~> 1.0)
      json (~> 1.6)
      mime-types (~> 1.16)
      multi_json (~> 1.0)
      multipart-post (~> 1.1)
      twitter-stream (~> 0.1)
    treetop (1.4.15)
      polyglot
      polyglot (>= 0.3.1)
    turn (0.9.6)
      ansi
    twitter-stream (0.1.16)
      eventmachine (>= 0.12.8)
      http_parser.rb (~> 0.5.1)
      simple_oauth (~> 0.1.4)
    tzinfo (0.3.39)
    uglifier (1.2.3)
      execjs (>= 0.3.0)
      multi_json (>= 1.0.2)
    unicode_utils (1.3.0)
    unicorn (4.2.0)
      kgio (~> 2.6)
      rack
      raindrops (~> 0.7)
    websocket (1.0.6)
    whenever (0.7.3)
      activesupport (>= 2.3.4)
      chronic (~> 0.6.3)
    will_paginate (3.0.3)
    xpath (1.0.0)
      nokogiri (~> 1.3)

PLATFORMS
  ruby

DEPENDENCIES
  active_attr
  authlogic
  capistrano
  capybara
  capybara-webkit!
  coffee-rails (~> 3.2.1)
  exception_notification (= 3.0.0)
  jquery-rails (~> 2.0.1)
  launchy
  letter_opener!
  mail_view!
  marginalia
  meta_request (= 0.2.1)
  minitest-rails
  minitest-rails-capybara
  nokogiri
  pg
  premailer!
  premailer-rails3
  psych
  quiet_assets
  rails (= 3.2.17)
  sass (= 3.2.6)
  sass-rails (~> 3.2.3)
  sheets
  snapshot!
  turn
  uglifier (>= 1.0.3)
  unicode_utils
  unicorn
  whenever
  will_paginate

¿Qué causa estos errores?