Never hashCode on a mutable value Revisited

In the "Never hashCode on a mutable value" post, I talked about the pitfalls of using a mutable value in your hashCode/equals computation. Another subtle variant of this is using a Comparator for a mutable value in a TreeSet (or other similar data structures). Because the set is sorted internally, and optimized based on that sort information, if you change the sorted value of one of your objects in the set, it can "disappear" if you attempt to .contains(..) check it later. I wonder how many more of these stupid mistakes I'll find ...