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I'd slap some big Adsense on it and enjoy!   I believe that one off topic page will not hurt anything - especially if the page is an outstanding example of content for its topic. I know sites that publish on a wide range of content and have fantastic rankings.  These sites were once single topic,  then departed with a single page that struck gold - and then started plowing that field into a hundred pages,  and soon ranking #1 for the single-word root keyword.    They then hit gold on a second topic, and a third.  Now these sites have diverse branches but have found ways to interrelate the topics. I don't think that Google has a problem with conflation.  You might even make Google think that the two topics are complementary.