In one of those science puzzles the general public has generally been sheltered, for whatever reason, the consensus has admitted problems did exist with moving spacecraft about by using gravity of big planets such as Jupiter to provide a boost. In the event, they claim they have solved the puzzle. It couldn't have been much of a problem as spacecraft were regularly boosted by the so called 'sling shot' method, and NASA has been very successful. It does show that a linchpin of mainstream, gravity, actually possesses a few quirks, or something else is going on they do not quite understand.
At http://tallbloke.wordpress.com/2013/10/12/variation-in-slingshot-effect-… … is a graphic description of the sling shot method and the news that the anomaly is not constant but is only apparent on the odd occasion. Mostly, the sling shot method is successful – without a blip. An ESA spokesman says they have been aware of the anomaly since 2005 (but it was absent in 2007 and 2011 for some reason). In fact, NASA noticed it as early as 1990.
At www.sciencedaily.com/releases/2013/10/131009111111.htm … you can find the same story.