I was not signed into the site when I got this error.
When I open any question on the Stack Overflow site, if I open the browser console, I keep getting an error message being continuously printed.
On Chrome, this is what it says
Uncaught Error: Error calling method on NPObject. File - B7924596.5;sz=728x90;ord=1385842058148:57
I was getting the same error even in incognito mode.
On Firefox, it says that there's an error on file/line number - 20471.js (line 14)
Here's the question where I first saw the error
Not sure if others are facing the same issue.