Getting a 404 for one blog post

Hi,

My blog page is here:

https://www.playshakespeare.com/blog?start=10

When I click any of the titles except one, they all go to the post page. For example, the first one:

https://www.playshakespeare.com/blog/entry/how-do-we-measure-measure-for-measure

But the fourth post down on that page title "Introductions" leads to a 404 error when clicking on it. Is this a bug?

Thanks!