The following problem is quite exceptional but worth mentioning: If you clone a subfield with a plugin, the plugin of the clone [often..]operates on the value of the first subfield. This has a few causes: 1) Instead of using the id parameter of the javascript function, the code in the plugin uses a perl variable in the javascript code. The cloned id is passed to onclick but ignored. 2) The Blur, Focus and Change event attributes are not even modified when cloning a subfield. Both things are true in e.g. marc21_leader.pl. If you connect this plugin to say 040d and you clone the field, you will see that clicking the second plugin operates on the first value. If you clear both fields, the Focus of the second field will fill the first field.
Working on report 10480, I will address this problem too by making a few changes more in cataloging.js.
Closing this due to age and it sounds like bug 10480 resolved the issue