the file share/html/Ticket/Modify.html changed in such a way that my "Default" callback was actually breaking custom field validation
Not so simple: I removed all the "local" directory, including plugins, stopped apache, cleaned mason obj, and mandatory CF are still not validating. Maybe it's something in 4.0.9. Being an outdated version, I will not annoy you anymore about that.
Regards, Bergonz -- RT Training - Dallas May 20-21 http://bestpractical.com/training