Call it J.C Penney’s private flash crash. The stock today plunged 19.7% to close at $26.57. That’s even worse than Black Monday 1987. On that day, the stock lost a hair less, 19.2%. It’s unclear if this is the stock’s worst day ever. (Perhaps it suffered an even worse day during the 1930s.) However, it is clear that this is the worst single-day performance in the past 40 years. That’s as far back as our FactSet data goes.*