This question on Stack Overflow required 4 votes to close, despite the requirement on Stack Overflow being only 3 votes to close a question. A similar bug was fixed and marked status-completed in 2010 (though the comments note its reappearance).
I suspect this was caused by a server error partway through the request for the final close vote. When I cast the third close vote, I received an error (unfortunately, I didn't take note of it at the time). It doesn't appear to be an issue of the last two close votes coming in simultaneously: some time elapsed between the third and fourth votes, allowing me to click around and refresh a few times before the last vote was cast.
Here is what the question looked like after I cast the third close vote and refreshed the page:
And here is what it looked like after someone else cast the fourth, closing it:
