By Josephine Mason Reuters November 21, 2012 NEW YORK, Nov 21 (Reuters) - A combination of short memories and old coffee has fueled an unprecedented sell-off in benchmark arabica futures this week, as changes made two years ago to the age and quality of beans deliverable against the ICE Futures contract finally come into effect. On Tuesday, the day before the first notice day for December coffee futures , that contract plunged over 5 percent and its discount versus March beans flared out to almost 9 cents per lb, the largest in 18 years, as traders who were long the contract rushed to dump it rather than roll into the next month. Rolling would mean selling the discounted spot and buying March at a big premium. While the spot prices recovered slightly on Wednesday, the big discount to March remained in place at 8.4 cents. A major reason for the selling of December is because new rules come into effect with the March contract, which will hike ...