I had some fun debugging a TreeSet once, as I was not aware of this information from the API:
Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the set, equal. The behavior of a set is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Set interface.
http://download.oracle.com/javase/1.4.2/docs/api/java/util/TreeSet.html
Objects with correct equals/hashcode implementations were being added and never seen again as the compareTo implementation was inconsistent with equals.
