The Data Persistence Error is an event that happened on May 28th 2014. This ROBLOX update erased everyone's saved data because of a bug. Here is a blog post from the ROBLOX Blog about the matter.
This update was recived negativly. Lots o' trainers quit because they lost all their Pokemon, many more got annoyed and also left. Trainers that believe to be the best and want to decided to stay and