Curtin version information:

$ dpkg -l | grep curtin
ii  curtin-common                              0.1.0~bzr470-0ubuntu1~16.04.1                 all          Library and tools for curtin installer
ii  python3-curtin                             0.1.0~bzr470-0ubuntu1~16.04.1                 all          Library and tools for curtin installer

Installation log:

$ maas admin node-results read system_id=xr36pd result_type=1 | grep "\"data\"" | cut -d"\"" -f4 | base64 --decode
curtin: Installation started. (0.1.0~bzr470-0ubuntu1~16.04.1)
Failed to exclusively open path: /dev/sda1
Traceback (most recent call last):
  File "/curtin/curtin/block/__init__.py", line 773, in exclusive_open
    fd = os.open(path, os.O_RDWR | os.O_EXCL)
OSError: [Errno 16] Device or resource busy: '/dev/sda1'
Device holders with exclusive access: []
Device mounts: ['/dev/sda1 /media/root-ro ext4 ro,relatime,data=ordered 0 0']
[Errno 16] Device or resource busy: '/dev/sda1'
curtin: Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'block-meta', 'custom']
Exit code: 3
Reason: -
Stdout: Failed to exclusively open path: /dev/sda1
        Traceback (most recent call last):
          File "/curtin/curtin/block/__init__.py", line 773, in exclusive_open
            fd = os.open(path, os.O_RDWR | os.O_EXCL)
        OSError: [Errno 16] Device or resource busy: '/dev/sda1'
        Device holders with exclusive access: []
        Device mounts: ['/dev/sda1 /media/root-ro ext4 ro,relatime,data=ordered 0 0']
        [Errno 16] Device or resource busy: '/dev/sda1'
        
Stderr: ''

Curtin machine config:

$ maas admin machine get-curtin-config xr36pd
Success.
Machine-readable output follows:
apt:
  preserve_sources_list: false
  primary:
  - arches:
    - default
    uri: http://nessus.rodsbooks.com/ubuntu/
  proxy: http://172.24.124.1:8000/
  security:
  - arches:
    - default
    uri: http://nessus.rodsbooks.com/ubuntu/
debconf_selections:
  maas: 'cloud-init   cloud-init/datasources  multiselect MAAS

    cloud-init   cloud-init/maas-metadata-url  string http://172.24.124.1:5240/MAAS/metadata/

    cloud-init   cloud-init/maas-metadata-credentials  string oauth_consumer_key=EUDbzueWEvEUz3XsUS&oauth_token_key=Y3MUmumkezRCgtTM93&oauth_token_secret=zuseqQu9bvTHSBJN3wwDphFskTpff2FU

    cloud-init   cloud-init/local-cloud-config  string apt:\n  preserve_sources_list:
    false\n  primary:\n  - arches: [default]\n    uri: http://nessus.rodsbooks.com/ubuntu/\n  proxy:
    http://172.24.124.1:8000/\n  security:\n  - arches: [default]\n    uri: http://nessus.rodsbooks.com/ubuntu/\napt_preserve_sources_list:
    true\napt_proxy: http://172.24.124.1:8000/\nmanage_etc_hosts: false\nmanual_cache_clean:
    true\nreporting:\n  maas: {consumer_key: EUDbzueWEvEUz3XsUS, endpoint: ''http://172.24.124.1:5240/MAAS/metadata/status/xr36pd'',\n    token_key:
    Y3MUmumkezRCgtTM93, token_secret: zuseqQu9bvTHSBJN3wwDphFskTpff2FU,\n    type:
    webhook}\nsystem_info:\n  package_mirrors:\n  - arches: [i386, amd64]\n    failsafe:
    {primary: ''http://archive.ubuntu.com/ubuntu'', security: ''http://security.ubuntu.com/ubuntu''}\n    search:\n      primary:
    [''http://nessus.rodsbooks.com/ubuntu/'']\n      security: [''http://nessus.rodsbooks.com/ubuntu/'']\n  -
    arches: [default]\n    failsafe: {primary: ''http://ports.ubuntu.com/ubuntu-ports'',
    security: ''http://ports.ubuntu.com/ubuntu-ports''}\n    search:\n      primary:
    [''http://ports.ubuntu.com/ubuntu-ports'']\n      security: [''http://ports.ubuntu.com/ubuntu-ports'']\n

    '
install:
  log_file: /tmp/install.log
  post_files:
  - /tmp/install.log
kernel:
  mapping: {}
  package: linux-generic
late_commands:
  maas:
  - wget
  - --no-proxy
  - http://172.24.124.1:5240/MAAS/metadata/latest/by-id/xr36pd/
  - --post-data
  - op=netboot_off
  - -O
  - /dev/null
network:
  config:
  - id: enp0s25
    mac_address: f0:92:1c:5e:1e:70
    mtu: 1500
    name: enp0s25
    subnets:
    - type: dhcp4
    type: physical
  - address:
    - 172.24.124.1
    search:
    - maas
    type: nameserver
  version: 1
network_commands:
  builtin:
  - curtin
  - net-meta
  - custom
partitioning_commands:
  builtin:
  - curtin
  - block-meta
  - custom
reporting:
  maas:
    consumer_key: EUDbzueWEvEUz3XsUS
    endpoint: http://172.24.124.1:5240/MAAS/metadata/status/xr36pd
    token_key: Y3MUmumkezRCgtTM93
    token_secret: zuseqQu9bvTHSBJN3wwDphFskTpff2FU
    type: webhook
storage:
  config:
  - grub_device: true
    id: sda
    model: FUJITSU MHV2060B
    name: sda
    ptable: gpt
    serial: NW84T6A2A9LH
    type: disk
    wipe: superblock
  - device: sda
    flag: boot
    id: sda-part1
    name: sda-part1
    number: 1
    offset: 4194304B
    size: 536870912B
    type: partition
    uuid: 2be68d8b-7e2c-44f1-b8b6-a0b80d076ea3
    wipe: superblock
  - device: sda
    id: sda-part2
    name: sda-part2
    number: 2
    size: 59466842112B
    type: partition
    uuid: b3a1130f-93d7-43a1-a9a6-a558a1e19985
    wipe: superblock
  - fstype: fat32
    id: sda-part1_format
    label: efi
    type: format
    uuid: 80bd9766-a8a9-49fb-943d-9da8c588b5ba
    volume: sda-part1
  - fstype: ext4
    id: sda-part2_format
    label: root
    type: format
    uuid: 49ec0137-d0bd-42eb-bb2e-858a4922e5f8
    volume: sda-part2
  - device: sda-part2_format
    id: sda-part2_mount
    path: /
    type: mount
  - device: sda-part1_format
    id: sda-part1_mount
    path: /boot/efi
    type: mount
  version: 1

Most of these failures have been on EFI machines, but at least one was on a BIOS machine. They're all configured for flat partitions, not LVM or bcache. The above output is for a system on my home network for which I'm using a local mirror, but I've seen failures on a MAAS server in 1SS that uses us.archive.ubuntu.com.