See this meta question.
The problem was that Stack Overflow was trying to access
When it should have done this:
I have been experiencing similar problems all over Stack Exchange. For example, here's one from math.SE (click image to enlarge):
Failed to load resource: the server responded with a status of 404 (Not Found)
And the URL that returned a 404 was
Not surprisingly, adding a slash worked:
I've been seeing these types of 404s on many other Stack Exchange sites.