If a Telemetry test that uses tracing fails in the middle of the tracing session (without crashing the browser), all subsequent Telemetry tests will fail (until the browser is restarted):

  Chrome
    ...
    Test 1
      Tracing.start successful
      Test fails without crashing the browser
      Tracing.stop is never called <------------------------+
    Test 2                                                  |
      Tracing.start FAILS because tracing is still running -+
    ...

This can lead to a chain of confusing failures:

  TracingUnexpectedResponseException: Inspector returned unexpected response for Tracing.start:
  {
    "id": 1, 
    "error": {
      "message": "Tracing is already started", 
      "code": -32603
    }
  }

Example: https://uberchromegw.corp.google.com/i/chromium.perf/builders/Android%20Nexus9%20Perf%20%283%29/builds/931 (there are a couple of actual failures in smoothness.key_mobile_sites_smooth.reference and all subsequent tests with device affinity 5 fail).