hong said:
"When in doubt, the DM's version of common sense takes precedence."
Yes. In-game and until the session is over.
This is part of the contract of playing in a DM's game: at the table, the DM functions as arbiter and judge, not just referee. If I'm DMing, and one of my players has a reasonable disagreement with my ruling that they can back up with facts
when the session is over, I will reconsider my ruling
for future sessions.
In addition, in cases of dispute, I will usually err on the side of the players. If there is a disputed case where one ruling means instant death for the player, and the other means survival, I'll err on the side of survival and/or allow the PC to take back his action.
PC: "I do X."
Me: "X is not something you'd survive. Would you like to reconsider?"
I'm also careful about placing any "instant death pits" in my set pieces. Threat of death? Sure. But there will always be a reasonable chance to avoid it...and it will be contingent on several failed die rolls, rather than just one.
For instance, being pushed over a deadly precipice would require bad positioning by the defender, a few successful strength (bull rush or grapple) checks on the part of the attacker, and the defender failing on a relatively achievable check to grab for the edge as they reach said precipice. Since the confluence of events is unlikely, the set-piece couldn't be considered unfairly lethal.
As a side note, the more likely a character is to fall in, the more survivable the fall should be. Or, to put it another way, only Gandalf should be plummeting off the bridge in Helm's Deep...