I knew a lady who was disfellowshipped for 20 years for smoking. She finally went back to the KH and since they couldn't find her paperwork, (all the former elders were dead) she was allowed back in without reinstatement. No one even remembered her. She had quit smoking years previously and could have gone back any time. She thought the shunning was permanent. She eventually became a Christian and was shunned permanently then though.
I have always wanted to talk to someone disfellowshipped (or divorced) for "extreme spiritual endangerment". Seems like that would cover just about anything you wanted it to.