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For those of you as stunned as I am that Word now marks two spaces as an error, there's how to fix it courtesy of Microsoft Answers

Although current convention is to use just one space when using proportional fonts (two spaces were used in typing because most typewriter fonts were monospaced), you can select which convention you want to use and have Word flag exceptions (or not). At File | Options | Proofing, beside "Grammar and Refinements," click Settings... In the Grammar Settings dialog, scroll down to Punctuation Conventions. You'll see that you can select one or two space or "don't check." As to why this just started, probably no one can tell you, but this is how to fix it.