This one is actually quite impressive, as opposed the dross usually disclosed within these threads.

During supersonic flight, the fuselage of the concord use to expand by around 30cm as a result of extreme external friction heating, i.e. the plane was roughly 1ft longer when in flight. This resulted in a gap appearing between the cockpit and the pilot's flight concole. On the final supersonic flights of one of the aircrafts, the pilots placed their caps in this gap, which on contraction during descent/landing were trapped there for all of time.