Previously there was a bug with many_to_many relationships that would improperly handing the unbinding of old relationships and not maintaining the new relationship when an entire many_to_many collection was replaced. Attempt at a coherent example below:

person.preferences == [ pref1.existing ]

person.preferences = [ pref2.new ]
person.preferences == [ pref2.new ]
pref1.people == [ person ] (wrong. it might have been right this long but a reload brought it back)
pref2.people == [ person ]

person.reload; pref1.reload; pref2.reload

person.preferences == [ ] (wrong, should have pref2)
pref1.people == [ person] (wrong, should no longer be related)
pref2.people == [ person ]

person.save; person.reload; pref1.reload; pref2.reload

person.preferences == [ pref2 ]
pref1.people == [ person] (wrong, should no longer be related)
pref2.people == [ person ]

Believe was the gist of it, but essentially it was getting into a half state between the old and new state.

Thanks,
Dave