Strange intermittent failures with Capybara. Always fails on CI server, sporadically fails in development environment, but never at the same assertion as on the CI server. Any clues why?

dashboard_spec.rb

describe 'dashboard' do
  it 'should be possible to edit an item', js: true do
    visit_subdomain 'bizeeme', '/dashboard/'
    # The browser app creates the UI with Backbone.js when loaded...
    within '#item-list' do  # <- It allways fails on the CI server here, because "#item-list" doesn't exist.
      find('a', text: /First/i).click
    end

    within '#content' do
      fill_in 'description', with: 'Changed description!'
      click_button 'Save'
    end
    
    visit '/items'
    page.should have_content 'Changed description!' # <- It intermittently fails here in the development environoment.
  end
end

rspec.log

  1) dashboard should show a list of items
     Failure/Error: within '#item-list' do
     Capybara::ElementNotFound:
       Unable to find css "#item-list"
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/result.rb:22:in `find!'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/finders.rb:26:in `block in find'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/base.rb:78:in `synchronize'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/finders.rb:26:in `find'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/session.rb:341:in `block (2 levels) in <class:Session>'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/session.rb:211:in `within'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/dsl.rb:51:in `block (2 levels) in <module:DSL>'
     # ./spec/frontend/dashboard/cms_spec.rb:8:in `block (3 levels) in <top (required)>'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara.rb:232:in `using_wait_time'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/dsl.rb:28:in `using_wait_time'
     # ./spec/frontend/dashboard/cms_spec.rb:5:in `block (2 levels) in <top (required)>'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:114:in `instance_eval'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:114:in `block in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:254:in `with_around_each_hooks'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:111:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:390:in `block in run_examples'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:386:in `map'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:386:in `run_examples'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:371:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block (2 levels) in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `map'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/reporter.rb:34:in `report'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:25:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `block in autorun'

When retrofitting wait_until and using that to wait for the element to become visible using:

def wait_until
  require "timeout"
  Timeout.timeout(Capybara.default_wait_time) do
    sleep(0.1) until value = yield
    value
  end
end

..the CI server fails with:

Failure/Error: wait_until { find('#item-list') }
     Timeout::Error:
       execution expired
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/base.rb:84:in `sleep'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/base.rb:84:in `rescue in synchronize'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/base.rb:77:in `synchronize'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/node/finders.rb:26:in `find'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/session.rb:341:in `block (2 levels) in <class:Session>'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/capybara-2.0.3/lib/capybara/dsl.rb:51:in `block (2 levels) in <module:DSL>'
     # ./spec/frontend/dashboard/cms_spec.rb:7:in `block (3 levels) in <top (required)>'
     # ./spec/frontend/frontend_helper.rb:40:in `block in wait_until'
     # ./spec/frontend/frontend_helper.rb:39:in `wait_until'
     # ./spec/frontend/dashboard/cms_spec.rb:7:in `block (2 levels) in <top (required)>'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:114:in `instance_eval'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:114:in `block in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:254:in `with_around_each_hooks'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example.rb:111:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:390:in `block in run_examples'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:386:in `map'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:386:in `run_examples'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:371:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block (2 levels) in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `map'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:28:in `block in run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/reporter.rb:34:in `report'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:25:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
     # /mnt/home/u1968/bundle/ruby/1.9.1/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `block in autorun'

I solved this one. It had nothing to do with Capybara. It was caused by a RequireJS path with an uppercase letter in it, while the actual filename was a all lower cased. This worked locally since it was running on Mac OS with the HD formatted as Mac OS Extended (Journaled) rather than Mac OS Extended (Case-sensitive, Journaled). The CI server was running Linux where the filesystem is case sensitive. This caused a HTTP error to occur when RequireJS tried to load the file in CI, while everything worked smoothly locally.
Since this shows up as an HTTP error in the browser console, but not in the browser's console log, it was a hard one to track down.