Mysterious "headers already sent" errors on Travis CI

While rolling the 2.0.2 release of Lightning, we encountered some extremely strange quasi-failures on Travis CI, such as you can see here: https://travis-ci.org/acquia/lightning/builds/188889997

This happened on several builds, persistently. We tried various things and spent a lot of time trying to figure it out, but we could not. I thought it might be a PHP bug -- a theory backed up by the fact that it stopped happening when I switched our Travis CI builds to PHP 7 -- but even after creating a Drupal VM environment on Ubuntu 14.04 (trusty) and compiling PHP 5.6.24 from source in order to test it...I could not reproduce it.

So we are currently living with this mysterious failure. Right now, we're kinda hoping that it's a Travis CI-specific issue that will eventually go away as the environments are changed and/or upgraded. But if anyone else is been able to reproduce this specific PHP error in their normal work, PLEASE post in here with the relevant info. We'd like to trace this if we can.

The specific error, as pulled out of Watchdog:

stdClass Object
  │  (
  │      [wid] => 107
  │      [uid] => 0
  │      [type] => php
  │      [message] => %type: @message in %function (line %line of %file) @backtrace_string.
  │      [variables] => Array
  │          (
  │              [%type] => RuntimeException
  │              [@message] => Failed to start the session because headers have already been sent by "" at line 0.
  │              [%function] => Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start()
  │              [%file] => /home/travis/build/acquia/lightning/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php
  │              [%line] => 140
  │              [@backtrace_string] => #0 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/Session/SessionManager.php(163): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start()
  │  #1 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/Session/SessionManager.php(118): Drupal\Core\Session\SessionManager->startNow()
  │  #2 /home/travis/build/acquia/lightning/vendor/symfony/http-foundation/Session/Session.php(71): Drupal\Core\Session\SessionManager->start()
  │  #3 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpFoundation\Session\Session->start()
  │  #4 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #5 /home/travis/build/acquia/lightning/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #6 /home/travis/build/acquia/lightning/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #7 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #8 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #9 /home/travis/build/acquia/lightning/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #10 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/DrupalKernel.php(652): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #11 /home/travis/build/acquia/lightning/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
  │  #12 /home/travis/build/acquia/lightning/vendor/drush/drush/commands/runserver/d8-rs-router.php(36): include('/home/travis/bu...')
  │  #13 {main}
  │          )
  ���  
  │      [severity] => 3
  │      [link] => 
  │      [location] => http://127.0.0.1:8080/history/5/read
  │      [referer] => http://127.0.0.1:8080/foobar
  │      [hostname] => 127.0.0.1
  │      [timestamp] => 1483545093
  │  )
  │  stdClass Object
  │  (
  │      [wid] => 108
  │      [uid] => 0
  │      [type] => php
  │      [message] => %type: @message in %function (line %line of %file) @backtrace_string.
  │      [variables] => Array
  │          (
  │              [%type] => RuntimeException
  │              [@message] => Failed to start the session because headers have already been sent by "" at line 0.
  │              [%function] => Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start()
  │              [%file] => /home/travis/build/acquia/lightning/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php
  │              [%line] => 140
  │              [@backtrace_string] => #0 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/Session/SessionManager.php(163): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start()
  │  #1 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/Session/SessionManager.php(118): Drupal\Core\Session\SessionManager->startNow()
  │  #2 /home/travis/build/acquia/lightning/vendor/symfony/http-foundation/Session/Session.php(71): Drupal\Core\Session\SessionManager->start()
  │  #3 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Symfony\Component\HttpFoundation\Session\Session->start()
  │  #4 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #5 /home/travis/build/acquia/lightning/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(99): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #6 /home/travis/build/acquia/lightning/docroot/core/modules/page_cache/src/StackMiddleware/PageCache.php(78): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #7 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #8 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #9 /home/travis/build/acquia/lightning/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #10 /home/travis/build/acquia/lightning/docroot/core/lib/Drupal/Core/DrupalKernel.php(652): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
  │  #11 /home/travis/build/acquia/lightning/docroot/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
  │  #12 /home/travis/build/acquia/lightning/vendor/drush/drush/commands/runserver/d8-rs-router.php(36): include('/home/travis/bu...')
  │  #13 {main}
  │          )
  │  
  │      [severity] => 3
  │      [link] => 
  │      [location] => http://127.0.0.1:8080/history/5/read
  │      [referer] => http://127.0.0.1:8080/foobar
  │      [hostname] => 127.0.0.1
  │      [timestamp] => 1483545098
  │  )
  │  

Postponing until it either vanishes forever, or someone else encounters it.