python2:

• Still in use: precise-truck-742:system_tests_160041019_1473898532
• Exists: precise-truck-742:system_tests_160041019_1473898532

then python3:

• Still in use: precise-truck-742:system_tests_160041019_1473898730
• Exists: precise-truck-742:system_tests_160041019_1473898730

From https://travis-ci.org/GoogleCloudPlatform/google-cloud-python/builds/160041019

Two proposed fixes:

• Make sure the 400 is caught and retried for "still in use"
• Use dataset IDs unique to each test (if possible)