So let's say you change phone numbers and FORGET to change one of the important websites that use that number for authentication? Or you change phones, wiping

So let's say you change phone numbers and FORGET to change one of the important websites that use that number for authentication? Or you change phones, wiping the old one before selling it to your friend and setting up the new one from scratch? Some websites are terrible/impossible at letting you recover your account when you've lost access to the phone number or the exact instance of the phone used for authentication.