Seems like some sort of filesystem corruption when it tried to mount the image, it also has an interesting error later on with an invalid binary package, maybe unrelated.

...
INFO    : The following images will be built            factory_install_shim.bin.
INFO    : Clearing shadow utils lockfiles under /build/setzer
INFO    : Verifying that the base image does not contain a blacklisted package.
INFO    : Generating list of packages for virtual/target-os-factory-shim.
INFO    : No blacklisted packages found.
INFO    : Using image type factory_install
INFO    : Using disk layout /mnt/host/source/src/overlays/baseboard-strago/scripts/disk_layout.json
WARNING: Primary GPT header is invalid
WARNING: Secondary GPT header is invalid
INFO    : Creating FS for partition 8 with format ext4.
INFO    : Creating FS for partition 12 with format vfat.
INFO    : Creating FS for partition 3 with format ext2.
INFO    : Creating FS for partition 1 with format ext4.
mount: wrong fs type, bad option, bad superblock on /dev/loop3,
       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=/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/factory_install_shim.bin target=/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/esp format=vfat ro/rw=rw options=offset=127926272
ERROR   : Tue Jun  5 20:27:35 PDT 2018
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 926:  '--from' '/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/factory_install_shim.bin' '--rootfs_mountpt' '/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/rootfs' '--stateful_mountpt' '/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/stateful' '--esp_mountpt' '/mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/esp'
ERROR   : Backtrace:  (most recent call is last)
ERROR   :  mount_gpt_image.sh:478:main(), called: mount_image  
ERROR   :  mount_gpt_image.sh:393:mount_image(), called: die 'Failed to mount all partitions in /mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/factory_install_shim.bin' 
ERROR   : 
ERROR   : Error was:
ERROR   :   Failed to mount all partitions in /mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/factory_install_shim.bin
INFO    : Unmounting image from /mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/stateful and /mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/rootfs
INFO    : Unmounting temporary rootfs /mnt/host/source/src/build/images/setzer/R67-10575.54.0-a3/rootfs//build/rootfs.
...
20:14:30: INFO: RunCommand: /b/c/cbuild/repository/chromite/bin/cros_sdk -- strip_package --board setzer chromeos-base/chromeos-chrome-67.0.3396.78_rc-r1 in /b/c/cbuild/repository
!!! Invalid binary package: '/build/setzer/packages/dev-only-extras.tbz2'
!!! Missing metadata key(s): CATEGORY, PF, SLOT. This binary package is not
!!! recoverable and should be deleted.
[05/Jun/2018:20:14:40] BUILD Filtering install mask from chromeos-base/chromeos-chrome-67.0.3396.78_rc-r1
pbzip2: *WARNING: Trailing garbage after EOF ignored!
20:14:54: INFO: RunCommand: /b/c/cbuild/repository/chromite/bin/cros_sdk -- equery-setzer list 'sys-kernel/*kernel*' in /b/c/cbuild/repository
20:15:01: INFO: RunCommand: /b/c/cbuild/repository/chromite/bin/cros_sdk -- strip_package --board setzer sys-kernel/chromeos-kernel-3_18-3.18-r2098 in /b/c/cbuild/repository
!!! Invalid binary package: '/build/setzer/packages/dev-only-extras.tbz2'
!!! Missing metadata key(s): CATEGORY, PF, SLOT. This binary package is not
!!! recoverable and should be deleted.
[05/Jun/2018:20:15:09] BUILD Filtering install mask from sys-kernel/chromeos-kernel-3_18-3.18-r2098
pbzip2: *WARNING: Trailing garbage after EOF ignored!
20:15:15: INFO: RunCommand: tar --sparse -I cat -cf /b/c/cbuild/repository/buildbot_archive/setzer-release/R67-10575.54.0/stripped-packages.tar /b/c/cbuild/repository/chroot/build/setzer/stripped-packages/chromeos-base/chromeos-chrome-67.0.3396.78_rc-r1.tbz2 /b/c/cbuild/repository/chroot/build/setzer/stripped-packages/sys-kernel/chromeos-kernel-3_18-3.18-r2098.tbz2
tar: Removing leading `/' from member names
20:14:20: INFO: RunCommand: tar --sparse -I /usr/bin/xz -cf /b/c/cbuild/repository/buildbot_archive/setzer-release/R67-10575.54.0/image_scripts.tar.xz partition_script.sh pack_partitions.sh umount_image.sh mount_image.sh unpack_partitions.sh in /b/c/cbuild/repository/src/build/images/setzer/latest-cbuildbot

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https://bugs.chromium.org/p/chromium/issues/detail?id=796254