Stagflation is the critical flaw in Keynesian economics. Keynes believed it could not happen. Effectively, stagflation is what happens when government starts increasing its spending during a depression - prices start rising (inflation), and the depression isn't reversed, partly because rising prices discourage additional spending. A more complicated, in-depth explanation would take a block of text you wouldn't care to wander through.
In short, Keynesian economics relies upon the idea that inflation is strongly associated with economic growth, and that if government engages in inflationary economic activities, it will spur economic growth. Th