After upgrading to 1.670 all of my File editors under Custom Commands and for instance editing the DHCP Server config are converting quotes and apostrophes to HTML equivalents.
So this line...
include "/etc/dhcp3/blocked-devices.conf";
would become...
include \"/etc/dhcp3/blocked-devices.conf\";
or didn't would become didn\'t
(minus the \ which seemed to be the only way to make the characters show up in this post.)
Operating system Debian Linux 7
Webmin version 1.670
This is a known bug in Webmin 1.670 which will be fixed in the next release.
If you need a fix sooner, the latest development version from http://www.webmin.com/devel.html already solves this problem.