There was a case in 1999 involving a Learjet which had departed Orlando bound for Dallas. At 1327:18Z, the pilot acknowledged Jacksonville ATC's clearance, "three nine zero bravo alpha." That was the last utterance heard from that airplane. At 1713Z, it dove into a field in South Dakota at nearly supersonic speed.
The NTSB determined that "The probable cause of this accident was incapacitation of the flight crew members as a result of their failure to receive supplemental oxygen following a loss of cabin pressurization, for undetermined reasons."
My stab at an MH370 theory:
Shortly after exiting Malaysian ATC, the captain locked the copilot out of the cockpit and turned left, intending to fly to some destination in the 'stans.
The alarmed copilot turned on his cell phone, which had been turned off prior to departure from Kuala Lumpur (there are records to this effect), hoping to call for help. But there was no service.
The copilot then arranged an assault on the cockpit. At some point, they passed close enough to a cell tower for the copilot's phone to check in. Aha! Service! (Reportedly, there are logs showing his phone checking in). But the copilot was by then too engaged in the struggle to notice.
Sometime later, the pilot, annoyed by repeated attempts to breach the cockpit, decided to subdue the passengers by depressurizing the passenger cabin. He figured the special oxygen supply for the flight crew would keep him alive.
But something went wrong. The depressurization gambit failed. The pilot's oxygen proved insufficient, and he went under along with everybody else.
At this point, the captain had steered the airplane in a southerly direction, intending to evade Indian radar. The autopilot maintained that flight path until the airplane ran out of fuel, and the plane crashed somewhere near where Inmarsat has calculated.