3D Systems (DDD -5.8%) spikes lower after Citron Research declares it a "bubble stock" in a new report (.pdf). The short-seller compares the current hype surrounding 3D printing to the dot.com, real estate, and alternative energy bubbles, criticizes 3D's low R&D spending and aggressive M&A strategy (so have some SA writers), and accuses management of providing a torrent of misleading promotional rhetoric. Also lower: SSYS -6.3%. XONE -2.1%. PRLB -7.7%.