ver 5.1 2503.
This one was tricky. When I would edit or add a certain record, I was getting NO validation happening although there was supposed to be. One of the errors from the browser was an ajax search suggest error. And this would break the page. Thus no validating. On this record one of the fields I hid was a picklist field setup as an ajax popup lookup. So I took this off in the editor so no more ajax suggest on the field. Rebuilt, that error was gone, but a new error displayed about the control still in the ajax_suggest. Back into the editor, opened properties of the lookup field. I also had set it as a dependant drop down. I took this off. Rebuilt and all worked fine with hiding the field. AND the validation magically kicked in. All is working fine now.
It something you might want to look into. Hiding fields based on group is necessary for me with my app. One should be able to hide any field without issue.