$mform->hardFreeze() does not seem to work for advcheckboxes used in an array

See: MDL-48047

We changed from advcheckbox to checkbox and it fixed our problem, but we did not find out what is wrong with advcheckbox.

The symptoms were that for form element 'abc[0]' calling setDefault('abc[0]', false) hardFreeze('abc[0]') resulted in getData returning abc[0] = 1 (sorry - if that's confusing).