Orbiter 2
Orbiter 2 was the first mission to use the Prototype 102 flight platform and the first catastrophic failure. Orbiter 2 was designed to test the 102 flight platform's viability to enter and exit orbit with parachutes added, one carried in the payload area; the platform was able to successfully enter orbit but due to a stall re-entering the atmosphere was unable to successfully land after exiting orbit and a failure occured at high speed.
Orbiter 2 began orbiting Kerbin at T+00:04:39 with the de-orbiting retrograde burn performed between T+00:56:19 and T+00:56:26. At T+01:08:38, Orbiter 2 re-entered the atmosphere, and at T+01:13:09, catastrophically crashed into the ocean, killing the onboard pilot.
The reason for Orbiter 2's failure was due to an unexpected, accidental triggering of staging in orbit which resulted in the fairings being removed earlier than expected as well as resulting in one of the engines being lost prematurely, suppressing Delta-V.