Do you have an error log on your web server? Does that only collect data that is essential or do you do something like log a referer or IP address or user id in hopes that it might be useful un debugging. Is there an access log? What happens if a user asks to delete their data. Do you go back and scrub all such logs? What about if you have a backend service that logs errors about what data it had problems accessing. Or an SQL server that logs bad queries?

It's not so straight forward as you make it seem.