Can't add my wife as an additional cardholder on my credit card, because she once used her phone number on a Barclays account of her own.
Having a rationale is good, contradicting real use-cases is not so much. I met validations that are constructed without a lot of cases in mind lots of times. My favourite one was assuming the zip code is always 5–6 numbers, which is not always the case in length and in some countries it's not numbers at all, and that was a logistical company that was supposed to know things well