[echelon]

IIRC, there was one commercial passenger flight where the captain locked himself out of the cockpit that led to everyone dying.

Have there been multiple separate incidents?

The other side of the coin is that hijackings used to be a frequent and regular occurrence. Now they're not anymore.

[danem]

You mean the incident where his copilot intentionally locked him out of the cockpit and crashed the plane into a mountain? Hardly seems like an indictment of locked doors to me.

[cyphar]

There was also Helios 522 where one of the cabin attendants only managed to enter minutes before the engines flamed out, there is a strong argument if the door wasn't locked he could've entered earlier.

And my understanding is that the current theory for MH370 is that the pilot locked out the copilot and then depressurised the cabin.

There are non-fatal cases like Ethiopian Airlines Flight 702 where the copilot locked the captain out when we was in the restroom (though loss of life was still a possibility, one of the engines had flamed out and was on emergency fuel).

As with all incidents, there are many factors that lead to them, but in these cases the presence of locked and reinforced cockpit doors contributed to the incident (in malicious cases the fact the door was impenetrable was clearly part of the decision-making, and in accident cases it was obviously an impediment to any positive outcome once the incident occurred).