From the Chicago Manual of Style Online's Q&A section:

Q. About two spaces after a period. As a U.S. Marine, I know that what’s right is right and you are wrong. I declare it once and for all aesthetically more appealing to have two spaces after a period. If you refuse to alter your bullheadedness, I will petition the commandant to allow me to take one Marine detail to conquer your organization and impose my rule. Thou shalt place two spaces after a period. Period. Semper Fidelis.

A. As a U.S. Marine, you’re probably an expert at something, but I’m afraid it’s not this. Status quo.

As someone who hates the two-space thing, I disagree with the marine, but I also wonder: Is this guy kiddding or what?