I know these threads come up every so often but here's my "I only just realised" one.
Just booked a few days in Yorkshire in August, started looking on the English Heritage website to see what we might want to visit and saw Harrogate. Which is not a seaside town. I have always thought it was. Maybe getting mixed up between a spa town and the seaside?
In my defence I did know about Portsmouth being an island but then to counter that I was also one of those eejits who saw the advert and thought Centre Parcs was all under the dome.