Operation Flash (or Spark depending on the translation). The March 1943 attempt to assassinate Hitler. An explosive was smuggled onto Hitler's plane, it went undetected, but the detonation device failed and the bomb didn't explode.

[Tresckow] asked Lieutenant Colonel Heinz Brandt who was traveling with Hitler if he would oblige to take a bottle of Cointreau to Colonel Helmuth Stieff (who was then not yet a conspirator) at Hitler’s headquarters in East Prussia as a payment for a lost bet. Brandt readily agreed. The “Cointreau” was actually a bomb constructed of a British plastic explosive “Plastic C” placed into the casing of a British magnetic mine, with a timer consisting of a spring which would be gradually dissolved by acid. Before Hitler’s Condor plane was to take off, Schlabrendorff activated the 30-minute fuse and handed the package to Brandt, who boarded Hitler’s plane. After takeoff, a message was sent to the other Berlin conspirators by code that Operation Flash was underway, which they expected to take place around Minsk. Yet when Hitler landed safely at his East Prussian headquarters, it became obvious that the bomb had failed to detonate (probably due to the extremely low temperature in the unheated luggage compartment thereby preventing the fuse from working). The message of failure was quickly sent out and Schlabrendorff retrieved the package to prevent discovery of the plot.

http://neoneocon.com/2010/12/13/hitler-assassination-plots-henning-von-tresckow/