https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1076619 reports:

The following sequence breaks /etc/group:

$ useradd 'vijai\'
$ usermod -aG sudo 'vijai\'
$ useradd blafasel
$ usermod -aG sudo blafasel

Results in:

sudo:x:27:install,vijaiaudio:x:29:install

Should have been:

sudo:x:27:install,vijai\,blafasel
audio:x:29:install

I believe this is caused by fgetsx "supporting" backslash as line-continuation indicator, but

• fputsx does not escape backslashes before line end
• there's a comment in fputsx indicating this is unsupported by everything else anyway.