looks like someone uploaded a bad file to BCS (again).  i don't have access to BCS or CPFE to resolve this myself.

https://luci-milo.appspot.com/buildbot/chromeos/coral-paladin/3503
https://logs.chromium.org/v/?s=chromeos%2Fbb%2Fchromeos%2Fcoral-paladin%2F3503%2F%2B%2Frecipes%2Fsteps%2FBuildPackages%2F0%2Fstdout

08:10:12: INFO: RunCommand: /mnt/host/source/.cache/common/gsutil_4.30.tar.gz/gsutil/gsutil -o 'Boto:num_retries=10' cp -v -- gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2 /var/cache/chromeos-cache/distfiles/target/Coral_EC.10068.63.0.tbz2.tmp
!!! Fetched file: Coral_EC.10068.63.0.tbz2 VERIFY FAILED!
!!! Reason: Filesize does not match recorded size
!!! Got:      117414
!!! Expected: 3664200

and indeed, looking at BCS, the file was messed up recently:
$ gsutil ls -al gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2
    117423  2018-06-04T07:55:36Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528098936119338  metageneration=2
   3664200  2018-06-04T08:59:31Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528102771464385  metageneration=2
    117414  2018-06-06T10:03:37Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528279417637199  metageneration=2
    117414  2018-06-06T10:44:57Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528281897875991  metageneration=2
    117414  2018-06-06T11:19:01Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528283941500542  metageneration=2
    117477  2018-06-06T11:35:53Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528284953241921  metageneration=2
    117414  2018-06-06T11:40:45Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528285245267463  metageneration=2
    117414  2018-06-06T11:51:55Z  gs://chromeos-binaries/HOME/bcs-coral-private/overlay-coral-private/chromeos-base/chromeos-firmware-coral/Coral_EC.10068.63.0.tbz2#1528285915985789  metageneration=2