Fields become "ghost fields"; they still show up in the sidebar on the right and you can still open their properties. However, you cannot select the form field and type text in it. You also can't change the properties, like name and tooltip fields; when you try to, and click away from the field you just changed, they revert back with no notification. This happens every time I open this file in a different application, such as an Accessibility plug-in called CommonLook, and has happened with other files too. Fixing tags in the plug-in causes forms to break in Acrobat, which means I have to go back and fix the form fields in Acrobat, which messes up the tags and tag order again... etc. etc.