London Bridge

You have most likely heard of London Bridge. People, regardless of country, think it is a majestic and architecturally-beautiful looking bridge that towers over the Thames (the river that winds through London) as it crosses the water. In reality, this is London Bridge:

http://en.wikipedia.org/wiki/File:London_Bridge_Illuminated.jpg

The bridge people often think is London Bridge is called Tower Bridge, and it is this one:

http://en.wikipedia.org/wiki/File:Tower_bridge_London_Twilight_-_November_2006.jpg

As with Big Ben/the Clock Tower, the name for one subject is being used to describe another, except in this case, it is two completely different structures having their names used wrongly rather than the name of an object inside a building being used to refer to the whole structure itself. I suspect with London Bridge/Tower Bridge, Tower Bridge is incorrectly called "London Bridge" because it is an icon of London like the Clock Tower, and is assumed to be the main bridge (hence "London Bridge"). Even when I went onto Google Images to find an image of London Bridge to link to, a large share of the images that came up were of Tower Bridge.