My wife and I travelled to France for a fortnight's holiday a few year's ago and whilst waiting for the ferry check-in desk realised that I had brought my own passport and my son's instead of my wife's.

It was too late to return home to collect the correct passport before the ferry sailed, so I just presented the two passports at the desk without comment - we were waved through, boarded the ferry, and spent the next fortnight wondering whether we would be stopped by gendarmes for a routine roadside check and if so what the consequences would be.

The holiday proceeded uneventfully, and even when we got back to Le Havre for the return crossing no comment was made on the dissimilarities in both personal details and photo appearance!