I would attack the guard instead and push his whistle up to his anus and say something cool like "Now blow the whistle out of your ASS!".

Game over!

Still the same, I would insert the whistle into the guard's anus. He is the culprit here, I hate the fucker already.

This kinda feels like something that would happen in an Elder Scrolls game.

A bit more seriously: that is the feeling I get in Rise of Nations Gold due to the 90 minute limit. No matter if I have already conquered 90% of enemy capitals and finally making progress in beating them away from the area, when that damn timer runs out of time, it is: "Nope, sorry, you lost the whole battle even though you were merely inches from winning it.". As if it doesn't suddenly matter at all that I conquered all those capitals, I still lost the whole area/fight.

It is not an action game but a RTS. And yeah it is the reason I stopped trying to play the game. Too bad, in theory it was "Age of Empires on steroids" which sounded just great.

The only difference being to your scenario that I know already beforehand there is some kind of time limit, while I guess in your example you are surprised for the fight to end abruptly, and not even sure why it was aborted.

I object to this for a very simple reason - I find this both immersion-breaking and nonsensical.
If we were going with any sort of realistic scenario, why would the bloody guard, if he's in a spot where he might possibly see you, not hear you fighting this ominous boss? If the boss fight is simply timed (for a logical reason) I wouldn't really mind.
"During the battle, you notice a guard. After a while, the guard looks at you" - well, what was the guard doing the whole effing time? Chewing his nails while his superior is getting his teeth kicked in? Either the guard notices the commotion right away or just happens to enter the room whilst you're in the midst of fighting. So, again, the scenario you describe is not simply bad game design imho, it's utter crap (or at the very least, hilariously bad AI).

I've definitely had this happen before - it's not that big a deal. Obviously not the exact scenario as put out by the OP, and I can't remember which games specifically it happened in but during my console years situations like this would certainly be a part of a lot of console action / action RPG type games.

As the first post describes, you'll be fighting a boss (or maybe not - it could happen when first entering a room - but I absolutely have vague memories of this kind of event during boss fights) and all of a sudden, game over. Before game over, you'll see a short cutscene - maybe it's a guard arriving, maybe it's that captured ally getting dropped into a vat of sharks with lasers on their heads, maybe it's a missle launching - usually accompanied by a pan over to a computer console in the room, or maybe a lever or some such.

After a few tries, you figure out you have to pull a lever / throw a switch / untie person X while fending off the boss, before the event in question has a chance to resolve (whatever - it all comes down to pressing X in front of object Y ;) ). Then you proceed as normal to beat the boss.

That gets me thinking of the ending to Beneath a Steel Sky, though it's a point-n-click so no fighting, but the timing is like that. Do nothing, you lose. Do something, and probably still lose. Do the right thing at the right time, and it'll work out. Yeah, it's the timing that reminds me of it.

Spiderman in the Sega Genesis had this scenario you describe. During your fight with the last boss (the Kingpin) Mary Jane is chained and being lowered into a vat of acid. If you take too long she's a goner and it is automatic game over. What you need to do is just to shoot web balls at the ceiling chain machinery from time to time in order to stop it from lowering her and then you can concentrate on kicking the kingpin's front teeth.
But I don't think it fits OP's specific scenario of awful design, because it's pretty obvious what you need to do to buy you time to defeat the boss.

I agree with what most people have said. That would be pretty bad design; and that is frankly quite obvious.

So what I don't understand is WHY the OP is asking about this specific scenario, unless they experienced it in a specific game, which doesn't seem to be the case.

She despises stealth sections in RPGs and is justifying that they are bad (not necessarily, but she is generally correct about it) but substituting even more inappropriate situations for stealth section tropes.

I'd shrug and do it right next time.

Ah, got it! Attempting to claim "Flawed by proxy". As if she needed validation to hate a particular game mechanic.

And here I was wondering how this thread would be spun into feminist attrition.

Sounds like awkwardly implemented stealth element. Stealth games was never my cup of tea, so I'll be probably pass. Nevertheless, they are quite popular genre, so nothing wrong in that. It can be sudden influx of real-world rules into the game: "so you thing you can have loud and long fight in General's office without guards coming there to check what was happening?". Done well, it might be brilliant - which isn't the case of your example of course. If there is a plausible physical obstacle, everything is OK. If the game simply doesn't allow you there then we got a "glass wall", old and clumsy (because obviously immerse-breaking) way to keep player character inside created environment. It is present in such a famous and successful games as Red Baron, Deus Ex or The Witcher. Tactical battlefields in wargames are mostly without any natural boundaries and gamers learn to deal with it. Generally speaking, if some game element became deal-breaker for me, I simply admit that I am not the target audience of the game, that me and the game "aren't fit for each other" and I stop playing it, because this game is no fun FOR ME. It proves nothing about the quality of the game in question, it can be great fun for somebody else.
If your point was "mixing different genre conventions into one game without warning is bad design by itself", it would be long and - I am afraid - futile discussion, because it boils down to "what kind of experience particular gamer wants".