Even the notoriously smarty pants "Countdown With Keith Olbermann" (and I say that with affection and admiration) makes a mistake that bedevils many of us. In a caption promoting Friday night's story about Sarah Palin's interview, the "Countdown" crew on MSNBC mixed up whose (possessive) and who's (contraction for "who is").

We have only one rule to remember: Possessive pronouns never use an apostrophe.