At times using and comparing apparently immutable data can cause significant disruption. Data changes, invariably. There is no think as fixed data, which is why in your database tables you have to use ID whenever possible, rather than data fields, to refer to records. But here I don't want to discuss the database theory, rather tell a couple of personal stories.
The first relates to my name. To be more precise, to my last name ( I didn't change name or ANYTHING like that!). The problem with my last name is that is has an accented latter. Now on my passport and ID card there no accented letter is possible, so my last name is spelled with a apostrophe after it. Not a big deal, you might think. However, on flight tickets only plain letters are used. That's why now and then as I board a plane I'm taken on a side by security claiming there is a problem because automatic check on my passport determined my ticket is for a different person. A rapid check unveils the issue, and I never had any serious problem.
Now let me get to the second story, the one I'm referring to in the title. Last year, the Italian postal service was so cute to determine that some middle-sized cities should now have two ZIP codes rather than a single one. So they changed my ZIP code. Again, it seems not a big deal, as mail with the original ZIP code will still get delivered for years. The problem is that my credit cards are tied to my ZIP code, so when I buy anything online and get it shipped I have to remember which ZIP code each credit card is bound to. If I get it wrong, the system will complain. I also had to remember the ZIP code of one of my credit cards to refuel while I was in the US recently.
Again, you might think this is not a big deal, as all I should do is edit my credit card information and update the ZIP code, right? Wrong: some of the sites used to edit that information won't let you enter the ZIP code but only pick the one for your city. And they have it wrong, so changing the data is far from trivial. Another system I use won't let me change my address without signing a specific paper requests, which is done (again) to avoid fraud. I've already been through it as not only I have this ZIP code issue, but the street I live in has two different spellings (San, for saint, vs. S.). So as I place an online order with my credit card, I might have it shipped to a different last name, at a different address, with a differetn ZIP code. But that's always me.
No, there is no such thing as safe immutable data!