The most common reasons for a sidebar to drop below the posts are that there are improperly closed div tags (each <div> needs to be closed with a </div> which should be inserted in an appropriate location) and a jump break <!-- more --> which has been accidentally inserted between a starting <div> and the ending </div>.
For more information about sidebar errors caused by jump breaks please see these two links: "Link 1" and "Link 2".
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UPDATED 22 November, 2009.
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