My son told me a riddle this morning that is a perfect illustration of the unique name assumption:

Two sons and two fathers went to a pizza restaurant. They ordered three pizzas. When they came, everyone had a whole pizza. How can that be?

I thought it was going to be a pun on the word whole, but it turned out to have a much better answer. I assumed that two sons and two fathers meant that there were four people. Of course there were only three: a grandfather, a father and a son.

This is something to watch out for when using OWL to represent a problem. OWL assumes non-unique names whereas in real life most people expect named things to be distinct. To model that assumption in OWL you have to explictly make owl:differentFrom assertions.