If you've ever played a pre-Wizards D&D, if you are part of the OSR, then you know you can't always win the fights you get into. It's a surprise for a lot of players that started with 3.x or 4e; like a JRPG those players sometimes assume that because something is present, the DM wants you to fight it. My evidence is anecdotal rather than statistic, but I've seen it dozens of times and heard stories of dozens more told by the very players who had that attitude. This article isn't to contest that one way or the other, but to talk about what player's should be prepared for.
If you feel sudden warning bells when you face something, perhaps because of the way it's being described or perhaps because it has just torn thousand-pound brass doors from their sockets and is brazenly storming a temple of a respected god, there are plenty of things you can do. Every loss doesn't need to result in being smeared to a fine paste of reddish meat upon the ground and walls, but the impetus to save your party must come from you.
As the song says, you need to know when to hold 'em and when to fold 'em. You need to make the decision to flee early, before anyone is down. Once someone falls, it becomes almost impossible to extricate yourself and you are either committed to trying to bring down whatever fell beast you're fighting or resigned to fleeing and leaving your downed comrade to die. Of course, just turning tail and running is dangerous as well since you might suffer several attacks to your back as you do. Spells and tricks that incapacitate your foes will come in handy here, allowing you the precious seconds to get the hell away. A well-timed grease spell can save the lives of your entire party.
But what are the circumstances that lead up to the run or die equation? There are only a two that can result in this and they are: bad dice rolls (either poor ones for your side or good ones for your foe) or bad preparation. Bad dice can always spell disaster, though it is rare and becomes rarer as you gain levels and your skills begin to make up for terrible rolls. However, the threat is always there. Even the easiest trouncing could turn to a slaughter in a few rounds if you aren't careful. That's one reason to always have an escape plan up your sleeve: no matter how powerful you are, watching your fighter take a critical hit to his temple will necessitate a withdrawal.
But ahh, player preparation... that is a different topic altogether. Most deaths are the result of poor preparation in battle. As I have told my players time and time again, if you are fighting fair (especially at low levels) you are begging to die. The dice do not favor you at the beginning of the game. Your skills are underdeveloped, your ability to stop your enemies often relies solely on a series of d20 rolls that are at best a 50-50 shot and at worst 1-in-20 affairs, not to even factor in creatures with immunity to mundane weapons! Before a battle begins, you need to do everything you can to stack the odds in your favor.
The more time you have to prepare for a combat, the better off you will be. I'm not sure if a group of level 1 PCs could fight a troupe of ogres in 3.x successfully (at least, using the rules as presented) but it is certainly possible in AD&D. Dig a trench around your camp, for example, giving you high ground when fighting the ogres who must cross it. Maybe put stakes out, forcing them to scramble over them and giving your archers and wizards key time to unleash hell upon them. Dig pit-traps, hire men-at-arms, or sneak into your opponents home and lay in wait. Ambush, sneak-attack, burning down buildings, all of these are ways to help ensure that combat goes in your favor.
The Dogs of the Exactor who were then comprised of Oloz, Crispus, and an elven fighter-mage who was not long for this world, have an example to help give you a sense of what kinds of things can be done to make a fight unfair. Working for Drozon the Cruel, they were then competing with other adventuring parties to discover the entrance to this secret city supposedly located in the sewers of Thurayn. They had fought and annihilated another adventuring party days before and learned, from their rapidly cooling corpses, of a band of desperados with a bad reputation: the Bloodletters.
Rather than waiting until the day they met the Bloodletters in the sewers and found themselves in dire straights, hammering against their skilled spearman, brutal dwarven cleric of Eridh the Crowfeeder, their angry wizard (who had fled the death of his first party to join them), and their extremely quiet and good-natured (but murderous) thief Fenwar, they concocted a plan.
They were only level one at the time, and they had a good inkling that the Bloodletters were beyond their pay-grade. However, they discovered the sewer-side entrance to the Bloodletter compound and resolved to use this information to murder them. This way, the battle would be on their terms, not in some tiny corridor where the Bloodletter wizard could unleash spell after spell and where they would have almost certainly died. Of course, the Bloodletter warehouse was trapped, and they had to deal with the scything blade that protected the sewer-side door, but once that was done they were ready.
The blade struck Oloz in the chest on their way in, and injured him badly. The Bloodletters weren't in, so they decided to wait for them. Figuring that most of their resources would be exhausted by the long crawl through the sewers their opponents were on, the Dogs were mostly fresh (save for a gravely wounded Oloz). The plan went as follows:
The Bloodletter "compound" was in reality an old stocking warehouse that had long been abandoned. Divided between its sewer-level cellar (and shrine to Fortuna) and its main open floor, the Dogs spread out to get themselves in optimal position. Oloz slumped against the wall near the sewer door, pretending to be dead from the blow of the scythe. The other two gathered at the top of the stairs so they could fight the Bloodletters one by one from a position of strength.
When the foe finally arrived, they inspected Oloz cursorily (the spearman pressed his spear into the recumbent form and Oloz, making a difficult CON check managed not to squirm or moan at the pain) and then moved on to go upstairs. At once, the trap was sprung. The elven fighter/mage unleashed a charm spell on the first person she saw: the Dorlish spearman. Oloz sprang to his feet and knifed their cleric in the back; his obscene strength coupled with his back-knifing skills cut her down at once. The enemy thief never entered, but the wizard was driven to his death by the spearman; once their charmed "ally" Daelus Swiftblade was asleep, they slit his throat.
While the beginning of this article was about handling a lose scenario, this is about avoiding one. Conventional wisdom says a level 1 party composed of three people cannot fight a four-man party composed of levels 2-4 characters; they'll not survive it based purely on an economy of hit-points and magic available. Yet, with some careful planning (knowing the makeup of the Bloodletters, finding their home, and using some tricksy tricks) they managed to destroy the Bloodletters without ever being injured.
That's one reason why I like these so-called no-win combats: good players will find a way.