A controversial research report about a prominent fintech was the torpedo that sank a clutch of the sector's stocks on Thursday. Two notable victims of this were insurance specialist Lemonade (NYSE: LMND) and next-generation lender Upstart Holdings (NASDAQ: UPST). The peer blasted with that report was Block (NYSE: SQ), which has drifted into the sights of a short-seller called Hindenburg Research.