test_problem_type_a11y fails infrequently due to a BadStatusLine: '' exception. This has been happening for a very long time. More details are available in this build.
This seems to be a known Selenium issue related to virtual browsers shutting down mid-test. There are some good diagnostic steps within this SO answer. There are a number of reasons why phantom might be dying in the middle of the test (machine OOMing, explicitly killing the process, etc).
To fix, I'd recommend running the a11y tests with a different virtual browser if possible. Phantom is currently unsupported and we'd do well to switch off it unless we have an explicit dependency. The selenium github thread linked above contains a number of good options. We explicitly set SELENIUM_BROWSER=phantomjs in accessibility_tests.sh, so this is a good spot to start experimenting with alternatives.
AC: test_problem_type_a11y no longer fails due to BadStatusLine on Jenkins. May be worth repeatedly rerunning the a11y job to ensure the flake no longer occurs.