A couple of years ago I flew in World Traveller from Sydney to London. I was in an aisle seat beside two guys going on a Working Holiday. They had a large bottle of rum with them from Duty Free. As I was on the aisle seat, I became galley runner to collect Cokes. We went through virtually the entire bottle on that flight.
One of the crew sniffed at me ruefully when I was getting more Coke at one point and I assured him all would be well. His reply was, "That's what they all say."
All was well and we were happily tipsy on arrival. Whether legal or illegal, it certainly happened with the knowledge of the crew in one form or another.