It appears users who went to a site that used a Facebook button to let users log in redirected to a Facebook error page. The glitch went away if users logged out of Facebook.com.
We experienced the glitch with several major sites including Gawker, New York Times, CNN, and even Business Insider.
The glitch only lasted a few minutes, and sites appear to be loading normally now.
Here's the statement from Facebook:
For a short period of time, there was a bug that redirected people logging in with Facebook from third party sites to Facebook.com. The issue was quickly resolved, and Login with Facebook is now working as usual.
Let's take a moment and digest this. Facebook is so ingrained in the Web that it can nuke just about every major site with one glitch. Not even Google has that kind of foothold.