Not the first time it's happened.
And as I've mentioned before, when you stay in a hotel it's the hotel's responsibility to register. You're responsible when you don't stay in a hotel. And booking online gives you proof where you stayed if there were a question.
(I remember a notice at one hotel a couple of years ago from the local PSB: If a hotel doesn't register you properly, contact us and we'll pay you a reward.)
I'm sure legally you'd be OK. But it could result in significant delays at the border should they raise a fuss. I have heard of a recent case where the traveller was delayed by two hours due to a missing registration from a hotel on a trip two years ago. They were sure that they had registered at the hotel but apparently it wasn't in the border force's system.