When the device is upgrade via USB procedure & license is added via "set system license keys key", then '/var/db/scripts/validator/device_serial_id' file will be missing.
This will cause commit error below while adding the license with CLI

[Errno 2] No such file or directory: '/var/db/scripts/validator/device_serial_id'

error: configuration check-out failed: (validation hook evaluation failed)

Engineering was engaged & fixed via below PR.
https://prsearch.juniper.net/problemreport/PR1783509

Issue:
During the USB upgrade, "/tmp/root/chas_ser_num.id" file does not exist. So the USB bootup commit causes failure.

Fix:
Added file existing check for ?/tmp/root/chas_ser_num.id? file, to avoid validation issue during USB upgrade.

In normal upgrade "/var/db/scripts/validation/" path is persistant , you will not see this issue.