Why Capybara feature tests randomly fail when using CI parallelisation?
It can happen that when you use CI parallelisation then your CI machine is overloaded and some of Capybara feature specs may randomly fail when tested website loaded slowly.
You can try to increase default Capybara max wait time from 2 seconds to something bigger like 5 seconds to ensure the Capybara will wait longer till the website is loaded before marking test as failed.
# spec/rails_helper.rb Capybara.default_max_wait_time = 5 # in seconds
For instance, this tip might be helpful for Heroku CI users who use Heroku dynos with lower performance.