Financial journalist Joe Nocera observed about the decision of the Federal Reserve and the Treasury not to help save Lehman Brothers:

Ever since that weekend, most people, including me, have viewed the decision by Henry Paulson Jr., the Treasury secretary at the time, and Ben Bernanke, the Federal Reserve chairman, to allow Lehman to go bust as the single biggest mistake of the crisis.

Why did the Treasury and the Federal Reserve allow Lehman Brothers to fail? Why do some consider the decision to be the biggest mistake of the crisis?