You live your life forward, but you understand it backward.
Immutability is one of those issues, like identifiers and introspection, that seems unimportant until something goes terribly wrong. Then, in the midst of the problem, you realize that your entire information system was designed incorrectly, and there really is nothing you can do to cope.
Here is an example of an immutability problem. You are a pathologist working in a university hospital that has just installed a new, $600 million ...