Example form: http://jquery.bassistance.de/validate/demo/milk/ (this is the demo for the script on bassistance.de)
When it does 'live' validation (on blur) it tells the user if the field is correct or incorrect. When you fill in the first field, tab to the next field, go back to the previous (allready filled in) field and clear it, it still counts as valid (notice the icon next to the field). Since the field is now empty, it should state as invalid, but it doesn't. This behaviour is random, sometimes it works, sometimes it doesn't. Sometimes it happens in the first field, sometimes on one of the other. It only happens before you try to submit the form, after a page submit everything works fine. Anybody who has a clue?