Any tabletop RPG player knows this can happen to the heroes or the villains. It doesn't matter how dramatic the story has made it, one lucky roll from either side can make a climactic showdown very, very brief. The extent to which this happens can tell a lot about the nature of a game and GM. Games that heavily avert this trope (such as Dungeons & Dragons and Star Wars D20) tend to create a very heroic, action-movie like feel.
Grittier, meaner, more brutal games (The World of Darkness, Dark Heresy, Call of Cthulhu, and so on) intentionally invoke this trope to help create the feel of danger, failure, and high stakes. Some games, such as the old West End Games D6 Star Wars adaptation, have rules written to invoke this trope and then blatantly tell the GM to lie and keep the PC's relatively safe, allowing them to feel like reality may ensue when it probably won't. Some games even shoot to overplay this trope in the name of schadenfreude; for instance, in Paranoia, your character is incompetent, your boss is insane, and your teammates will throw you under the bus at the drop of a hat— so sure enough, you're guaranteed to suck, fail, and die repeatedly for laughs.
An edition of Hackmaster averts No Arc in Archery by noting that shots at long enough distances need a high enough ceiling to not get in the way of the arrow's trajectory.
In-universe, this trope is a common lament of The Fair Folk in Exalted - Creation doesn't "play fair" and actually, well, enforces the consequences of their actions. In the Wyld, things work by dramatic rules, and a raksha can murder his friend, fall in love, or be eaten by tigers without actually needing to worry about the long-term effects. As a result, they are likely to be caught flat-footed when they walk into Creation and suddenly die, permanently, when they are killed.
Dark Eldar Wyches are dressed Stripperiffically... and have exactly the kind of piss-poor armour save you'd expect in reality.
The setting makes full use of taking place over an entire galaxy, which is a bloody huge place. Massive wars fought over decades and dozens of systems and cost billions of lives, like the Sabbat Worlds crusade, are ultimately minor affairs that have little to no effect on the galaxy as a whole. Even ten thousand years later huge swaths of the galaxy are still unexplored or barely understood, with new civilizations discovered (and exterminated) on a regular basis. It takes months or years to travel any significant distance even with (almost) reliable FTL travel. And while the Imperium is most certainly dying, it's so big that it will take thousands of years to be destroyed completely.
For all the equipment, training and propaganda supplied to them, most Imperial Guardsmen are absolutely aware of just how outclassed they are against the majority of their opponents. This means they will use any dirty trick in the book, including good old fashioned cowardice. The Imperium has a novel solution to this, however when they are insisted to fight bravely against impossible odds, the Guardsmen have been known to "insist" back.
Even without the spontaneity of any combat mechanic, there are situations where the DM may just call the Chunky Salsa Rule on you, regardless of what the numbers say.