Unbeknownst to us (or the hr system owner) after a software update it started sending us extra fields which would occasionally include the office managers name and contact details (including home address at times).
Had another case where a pos vendor was supposed to be sending us aggregate sales data. Again, not in the agreed spec, they included the names of any loyalty users who had used each pos.
I once saw a script kiddie spew a whole credit card file into a non existent endpoint filling the request logs with pci scoped data.
Many web based api (especially json based ones) will not refuse requests that have extra fields in them, so you can get in the case where poorly crafted clients are sending data they shouldn’t.
My point is many many systems don’t ask for personal data and still get and store it.