I think it was to commemorate 10 years of One Piece but im not sure.
Oda and Toriyama created a one off crossover called 'Cross Epoch' i don't know if anyone has read it yet but it's a very nice combination of characters especially Sanji and the old man (forgot his name) from DBZ
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!!!! As usual please don't post links to Scans or Translations of it !!!!