The 20 July plot. The Wikipedia article on it is pretty good if you want to read up on it. A bomb was detonated in a conference room with Hitler present, and he only survived because a table leg blocked a lot of the blast. Here's a picture of Hitler's pants after the blast - basically destroyed: http://upload.wikimedia.org/wikipedia/commons/9/95/Bundesarchiv_Bild_146-1972-025-64,_Hitler-Attentat,_20._Juli_1944.jpg