See for example:

https://luci-milo.appspot.com/buildbot/chromeos/betty-arc64-release/145

The first VMTest attempt fails because the device runs out of space in dev_mode_test.
However the failure in the second attempt is different:
"
08:11:16: INFO: Running VM test dev_mode_test.
08:11:16: INFO: RunCommand: /b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py --verbose betty-arc64 /b/c/cbuild/repository/src/build/images/betty-arc64/latest-cbuildbot/chromiumos_test_image.bin
2017/10/02 08:11:17 - devinstall_test.py - INFO    : Setting up the image /b/c/cbuild/repository/src/build/images/betty-arc64/R63-9995.0.0/chromiumos_test_image.bin for vm testing.
2017/10/02 08:11:17 - devinstall_test.py - INFO    : Making copy of the vm image /b/c/cbuild/repository/src/build/images/betty-arc64/R63-9995.0.0/chromiumos_qemu_image.bin to manipulate.
2017/10/02 08:11:27 - devinstall_test.py - DEBUG   : Copy of vm image stored at /tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/chromiumos_qemu_image.bin.
2017/10/02 08:11:27 - devinstall_test.py - INFO    : Wiping /usr/local/bin from the image.
2017/10/02 08:11:35 - devinstall_test.py - WARNING : Received error during cleanup
Traceback (most recent call last):
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py", line 73, in Cleanup
    self.vm.Stop()
AttributeError: 'NoneType' object has no attribute 'Stop'
Traceback (most recent call last):
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py", line 188, in <module>
    main()
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py", line 179, in main
    test.PrepareTest()
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py", line 106, in PrepareTest
    self._WipeDevInstall()
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/devinstall_test.py", line 87, in _WipeDevInstall
    safe=True)
  File "/b/c/cbuild/repository/src/platform/crostestutils/devmode-test/../../crostestutils/lib/mount_helper.py", line 23, in MountImage
    cmd, print_cmd=False, cwd=constants.CROSUTILS_DIR, capture_output=True)
  File "/b/c/cbuild/repository/chromite/lib/cros_build_lib.py", line 653, in RunCommand
    raise RunCommandError(msg, cmd_result)
chromite.lib.cros_build_lib.RunCommandError: return code: 1; command: ./mount_gpt_image.sh '--from=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest' '--image=chromiumos_qemu_image.bin' '--rootfs_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/m' '--stateful_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/s' --safe
mount: wrong fs type, bad option, bad superblock on /dev/loop1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

ERROR   : mount failed: device=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/chromiumos_qemu_image.bin target=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/s format= ro/rw=rw options=offset=4456448000
ERROR   : Mon Oct  2 08:11:35 PDT 2017
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 9410: ./mount_gpt_image.sh '--from=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest' '--image=chromiumos_qemu_image.bin' '--rootfs_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/m' '--stateful_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/s' '--safe'
ERROR   : Backtrace:  (most recent call is last)
ERROR   :  mount_gpt_image.sh:427:main(), called: mount_image  
ERROR   :  mount_gpt_image.sh:346:mount_image(), called: die 'Failed to mount all partitions in /tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/chromiumos_qemu_image.bin' 
ERROR   : 
ERROR   : Error was:
ERROR   :   Failed to mount all partitions in /tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/chromiumos_qemu_image.bin

Current loopback device status:
    /dev/loop0: [0804]:56887108 (/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/chromiumos_qemu_image.bin), offset 2308964352

cmd=['./mount_gpt_image.sh', '--from=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest', '--image=chromiumos_qemu_image.bin', '--rootfs_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/m', '--stateful_mountpt=/tmp/cbuildbot-tmpJYrHjH/tmp5k0K6WDevModeTest/s', '--safe'], cwd=/b/c/cbuild/repository/src/scripts
08:11:35: ERROR: 
!!!VMTests failed!!!
"

It looks as if the failure in the first attempt has left the system in a bad state that breaks (all?) further attempts.