Cramer said Supervalu teaches us two important lessons. First, the supermarket business is a hard one, with increased competition from Whole Foods ( WFM) at the high end and Target ( TGT), Wal-Mart and the dollar stores at the low end. But more important, Supervalu teaches the lesson that sky-high yields can be red flags, especially at companies in long-term spirals downward. Cramer said any investor doing their homework could have seen months ago that Supervalu's payments simply weren't sustainable, no matter what management was telling us. "Buy and homework" remains the lesson of the day, Cramer concluded. --Written by Scott Rutt in Washington, D.C. To contact the writer of this article, click here: Scott Rutt. To follow the writer on Twitter, go to http://twitter.com/scottrutt. To submit a news tip, send an email to: email@example.com. To watch replays of Cramer's video segments, visit the Mad Money page on CNBC.