I'm pretty sure that shouldn't happen - sudo/useradd '#0'

sudo-lolwut.log

root@debian-sid-testbed:/home/marshall/ansvif# useradd '#0'
root@debian-sid-testbed:/home/marshall/ansvif# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
systemd-timesync:x:101:102:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
systemd-network:x:102:103:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:103:104:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
messagebus:x:104:110::/nonexistent:/usr/sbin/nologin
sshd:x:105:65534::/run/sshd:/usr/sbin/nologin
marshall:x:1000:1000:Marshall Lee Whittaker,,,:/home/marshall:/bin/bash
systemd-coredump:x:999:999:systemd Core Dumper:/:/usr/sbin/nologin
#0:x:1001:1001::/home/#0:/bin/sh
root@debian-sid-testbed:/home/marshall/ansvif# sudo -u#0 su -
root@debian-sid-testbed:~# exit
logout
root@debian-sid-testbed:/home/marshall/ansvif#
root@debian-sid-testbed:~# id
uid=0(root) gid=0(root) groups=0(root)
root@debian-sid-testbed:~# exit
logout
root@debian-sid-testbed:/home/marshall/ansvif# sudo -u "#0" su -
root@debian-sid-testbed:~# exit
logout
root@debian-sid-testbed:/home/marshall/ansvif# sudo -u#0 su -
root@debian-sid-testbed:~# exit
logout
root@debian-sid-testbed:/home/marshall/ansvif# sudo -u#1001 id
uid=1001 gid=0(root) groups=0(root)
root@debian-sid-testbed:/home/marshall/ansvif# sudo -u#0 id
uid=0(root) gid=0(root) groups=0(root)
root@debian-sid-testbed:/home/marshall/ansvif#

This seems like its not so much a sudo problem as a problem that happens when a group is created with a name that some programs don't identify as legal. So since the environment is strange it inherits the old environment (gid 0).