The problem was several unclosed div tags in the http://nonprofiteer.net/2013/03/05/dear-nonprofiteer-is-the-banquet-deductible/ and http://nonprofiteer.net/2013/02/28/dear-nonprofiteer-when-is-a-board-member-not-a-board-member/ and http://nonprofiteer.net/2013/02/04/dear-nonprofiteer-if-i-smell-a-rat-should-i-just-hold-my-nose/ and http://nonprofiteer.net/2012/12/31/really-bad-advice-about-year-end-giving-and-some-really-obvious-responses/ posts, all interacting together in a very bad way.
They're gone now, but if you're copying and pasting content from elsewhere (not directly typing into the post editor), make sure that you paste into the Text editor tab, not the Visual editor tab.