If something being unit tested calls wfResetOutputBuffers(), it will remove the output handler that PHPUnit uses to catch output to detect tests producing output when they shouldn't be.
In the old version of PHPUnit used by Jenkins, this manifests as a warning "ob_end_clean(): failed to delete buffer. No buffer to delete" causing the test to fail. In more modern versions of PHPUnit, the missing buffer is detected and the test is instead marked as "risky".
Of course, it breaking depends on wfResetOutputBuffers actually working in the first place (see T109842 and [[https://github.com/facebook/hhvm/pull/4868|HHVM bug 4868]]).