I reproduced an error on a clean Drupal 6.14 install. Added Tabs, CCK Fieldgroup-tabs, filefield and imagefield.
Made some fieldgroup tabs and added one imagefield to one tab and a filefield to another tab.
When submitting an empty form. you will get the 'fieldx is required' error message. So far nothing wrong.
But when you then upload a file (either image or file), the tabs break as soon as the preview or file details are faded in.
Tabs are not clickable anymore and you are stuck

I am not a pro and i have spent days breaking my head.
But i found, that after deleting the line :

$(this).addClass('error').addClass('ui-tabs-selected');

from Tabs.js things work normal again.

For your info

Comments

I'm having the same problem! I have placed some forms in tabs and when an error occurs (ie empty form) the first tab's content gets the error class (which is not the tab with the error). If I remove the following code everything get's fixed but again the tab containing the error is not selected.

$('.drupal-tabs').children('ul > li').each( function() {
    if ($($(this).find('a').attr('href')).find('div.form-item .error:first').size()) {
      $(this).addClass('error').addClass('ui-tabs-selected');
    }
  });

Can anyone fix the above code in order to select automatically the correct tab and mark it as having an error?