Seen once on a rammus-release:

https://luci-logdog.appspot.com/logs/chromeos/buildbucket/cr-buildbucket.appspot.com/8925057776721731632/+/steps/UnitTest/0/stdout

First, it looks like attestation unit tests succeed, but fail to cleanup at the end:

attestation-0.0.1-r2529: rm: cannot remove '/build/rammus/etc/passwd.lock.32568': No such file or directory
attestation-0.0.1-r2529:  * ERROR: chromeos-base/attestation-0.0.1-r2529::chromiumos failed (preinst phase):
attestation-0.0.1-r2529:  *   Failed to lock /build/rammus/etc/passwd.lock.
attestation-0.0.1-r2529:  * 
attestation-0.0.1-r2529:  * Call stack:
attestation-0.0.1-r2529:  *     ebuild.sh, line  124:  Called pkg_preinst
attestation-0.0.1-r2529:  *   environment, line 3742:  Called enewuser 'attestation'
attestation-0.0.1-r2529:  *   environment, line 1632:  Called egetent 'passwd' 'attestation'
attestation-0.0.1-r2529:  *   environment, line 1465:  Called _portable_grab_lock '/build/rammus/etc/passwd.lock'
attestation-0.0.1-r2529:  *   environment, line  628:  Called die
attestation-0.0.1-r2529:  * The specific snippet of code:
attestation-0.0.1-r2529:  *       rm "${lockfile_1}" || die "Failed to lock ${lockfile}."

Then, many other tests start failing:
arc-appfuse-0.0.1-r85: error: could not grab lock /build/rammus/etc/passwd.lock
...
arc-appfuse-0.0.1-r85: OSError: [Errno 17] File exists