Warhammer, particularly in later editions. The Empire have a steam-powered tank and a "clockwork" horse while the Dwarfs - better yet - have a helicopter armed with a steam cannon (not mentioning an organ gun and a huge cannon-like flamethrower). The Skaven, infamously, feature "fantasy" versions of a sniper rifle, a ratling gun, a flamethrower, a laser cannon, a hamster wheel of death and what what appears to be three separate types of nuclear bombs-including a Davy Crockett Personal Nuclear Missile Launcher (all of which may fail with destructively hilarious results). This in a world where a powerful human kingdom still think knights and longbows are cutting-edge, and there's at least one major faction that's entirely Stone Age. Fan reactions have been mixed, although some earlier editions featured actual plasma guns and laser pistols, so modern players get off lightly really. The schizo tech contraptions tend to go haywire in the most spectacular ways imaginable at the moment least desired. But even then they are fun to play - if not, just for laughs.
Due to a bit of "lost tech" going on, the Imperium's technology is controlled by a religious cult called the Adeptus Mechanicus that doesn't believe in researching new technology or trying to understand the technology they currently have. Instead, they worship whatever old technology they can find from before the Age of Strife.
As a more straight example, the Space Marines have floating bio-mechanical skulls called servo-skulls, basically a cybernetic computer that can hover. They put candles on them when they need some extra light.
The Imperial Guard can sometimes be a good source of this. Example: the Leman Russ Executioner battle tank. It looks boxy and crude, the engine's designed to run on anything you can burn up to coal and wood, and the heavy stubber on top is pretty much a World War II-era M2 Browning in all but name, but it packs a huge tank-melting plasma cannon for the main gun. An Imperial Guard force is probably the only place where you'll find motorcycle troops and horse cavalry fighting alongside Sentinels and Warhound Titans.
Despite popular belief, the Astra Militarum has no "official" loadout enforced across all regiments; individual regiments are largely responsible for providing their own equipment and training. The image of Guardsmen all wearing green, angular armour comes from many regiments copying the famed Cadians. Since planets in the Imperial Guard vary wildly in technology levels, you get a lot of this when regiments are folded into each other or co-operate: you may get professional troopers from a Forge World equipped with plasma weapons and utilising cameleoline cloaking and cybernetic augmentations, fighting alongside Feral World primitives who like draping themselves with noxious body paint and the bones of dead comrades, and prefer tomahawks and longbows over their lasguns. It sounds crazy but this is a necessity for an interstellar empire that is so profoundly large, ancient and thinly-spread that it doesn't even really know how many planets it controls.
Also very noticeable in spaceships which make extensive use of manual labor. So you have a multi-kilometer long spaceship powered by an advanced plasma reactor where the multi-story tall shells for the guns are loaded by hand using ropes and pulleys (and whips).
Most races actually manage to avoid this trope fairly well, with Eldar, Necrons, Tau and Tyranids having reasonably consistent technology levels. Orks, on the other hand, really don't. Primitive axes, clubs and boar-riding cavalry are regularly seen alongside laser guns and Humongous Mecha. The in-universe explanation is that their technology works by the Rule of Cool – if Orks believe something they've built will work, it will.
Exodite Eldar intentionally invoke this. Their Dragon Knight warriors ride dinosaur-like creatures, and they have laser lances (much like the Shining Spears aspect warriors but more primitive) and laser carbines (much like Imperial lasguns except they don't suck). Exodites limit their technology in such a way to give themselves hard lives to stop their culture from falling into decadence and depravity; otherwise, they have almost exactly the same technology as their craftworld cousins.
While the Tau avoid this trope like the plague, their Kroot allies do this intentionally. They keep tech for the most part very simple (they can absorb the DNA from other species), but still have space travel and their otherwise primitive guns fire incredibly advanced munitions given to them by the Tau.
The Tau Farsight Enclaves' tech is somewhat dated compared to the Tau Empire's tech. They do occasionally get some shiny new toys thanks to spycraft, theft, and/or sympathizers within the Empire. Farsight himself still uses the same old battlesuit he wore when he first left the Empire.
Being set AfterAfter the End, the game has a lot of this. Many wilderness villages may not have running water and only a few electrical generators, but will have laser rifles capable of blowing a sedan in half with one shot. And let's not get into magic.
The Coalition States uses this to their advantage to peacefully assimilate human communities.note They generally assimilate non-human communities by flattening them and claiming the smoking crater as their territory. They offer to help Low Culture, High Tech towns to perform repair and upkeep on their technology, and use that to make the town more and more dependent on the Coalition, until they quietly absorb the community into their empire.
BattleTech is also rife with this sort of thing. The mecha all run on highly compact and portable fusion engines and have guns and missiles with great range and hideous damage, but due to the rubbished industrial base apparently nobody can build decent fire control or air-conditioning systems, so most fighting takes place at close range (under 1 kilometer!) and most mechwarriors fight in what amounts to underwear. (Admittedly, the short ranges are both to keep map sizes reasonable for gameplay purposes and because the designers were shooting for a classic in-your-face mecha combat aesthetic in preference to more logical but boring long-range sniping contests.)
Similarly, while BattleMechs have essentially taken over the role of tanks — not necessarily combat vehicles in general, just the old twentieth-century style armored boxes with a gun turret — centuries ago in-universe, everybody still uses those anyway. Some handwaving about how they're supposedly cheaper and easier to produce is basically canon, and of course weapons and armor have kept up with the times, but it's still rather akin to keeping prop fighters in production and actual military use long after everybody already knows how to make perfectly good jets. Tanks are still used because they're comparatively cheaper and stealthier. They lack the technology to keep Humongous Mecha from being ludicrously expensive artillery bait (which is probably why the Clans' Obstructive Code of Conduct forbids artillery).
Schizo tech comes up a lot when dealing with backwater colony worlds. A hunter might use a black powder pistol to kill a deer for dinner, then come home and cook it in a microwave. At least part of all of this is due to the general devastation of and Inner Sphere-wide technological backslide during the Succession Wars.
Quite a few of these are very subtle ones that most people probably wouldn't notice. While much of the technology is medieval level at the latest, windows using 19th-century sheet or plate glass are described, chimneys (a 16th-century invention) are universal, and even seemingly-minor details such as the existence of "private rooms" at inns are all developments of later centuries. On the flipside, pre-Dark Age technologies such as triremes, chariots, colosseums and the notion of daily bathing also appear.
Gary Gygax played around with this trope a lot in his original Greyhawk home games, although most of them (mostly imported from Earth or found in crashed spaceships) got left out in later releases for that campaign setting.
The Hollow World, inside Mystara, proactively averts this trope with the Spell of Preservation, which makes people in various cultures distrust and spurn unfamiliar technologies, no matter how useful. (Of course, some of those cultures may have Schizo Tech of their own, but they won't be developing any new examples while under this spell.)
Technology levels in Ravenloft range from Stone Age to late Renaissance, depending on where you are, with even higher tech turning up in the local Mad Scientist Laboratory. This is because new domains are added to the Land of Mists from different worlds with their own indigenous tech-levels, rather than technology evolving in tandem within adjacent countries.
The Dragonlance setting has the Tinker Gnomes who power their dormant volcano home with Geothermal power and individual Gnomes have invented things like Powered suits of armor, Invisibility Spray, various Clockwork automaton, and even a nuclear bomb. The Tinker Gnomes are a race of bungling inventors, and so a lot of their technology does tend to be a bit prone to exploding.
Spelljammer is about as close to the franchise gets to a Standard Sci Fi Setting, and the mechanics of spelljammer helms mean that just about anything can get into space while gunpowder weapons are considered a bad idea. Ships in the setting range from "Roman galley with a spelljammer helms strapped to it" and "armored warship with lasers," and one prominent faction equips some members with spring-loaded dart guns and others with lightning cannons.
The world Golarion is deliberately all over the place. The Inner Sea Region contains every conceivable level of technology, from simple tribal cultures with axes and spears, to the standard fantasy kingdoms policed by knights on horseback, to a nation where the wreckage and technology of a fallen city-sized spaceship litters the land of savage barbarians. To top it all off, a steampunk city with black powder firearms is wedged into a magical wasteland between two nations dominated by wizards. The designers actually acknowledge the trope in one book, citing Truth in Television: not every nation develops at the same rate or obtains new technologies in the same order.
One adventure path, Reign of Winter, specifically enforces this as a point in artwork and theme. The adventure eventually brings you elsewhere in the universe at the modern day of the game: Earth, around 1912 C.E., where the adventurers encounter Great War-era Russians. The game states that while the sudden appearance of armored knights and robed, flying elves may be unusual, the Russians had seen far worse threats during the war as they level their machine guns toward the new threats.
Spirit of the Century plays with this, as it's set in the 1920s but uses pulp Science! to allow more futuristic technology, and even full on mad science inventions that we still haven't made. The book does a good job of cataloguing what inventions are just around the corner to give you some idea what the state of the art inventions you could get prototypes to, or make, are.
There's SchizoMagiTech. Conventional technology is mostly around middle-to-late Bronze Age/early Iron Age. But those with the needed skills can create a hyper-precision wristwatch with perpetual calendar, sunrise and sunset calculator, moon phase display, and the functional equivalent of high resolution GPS as a minor tool.
Even without the inventors, Creation still has a variable tech level, ranging from cities where a few guards may have firewands and there's a medieval level civic works thing going on, to cities like Chiaroscuro where the rich quarters have elevators and a functional equivalent of electricity, to the relative metropoli of the Blessed Isle.
Exalted also has the Lost Technology angle going for its Schizo Tech. A lot of the more powerful or complex Artifacts are remnants of First Age technology made by Solar artisans. Of course, the Solar Exalted have spent most of the last couple thousand years being dead and have only just recently returned. Enough documentation has survived that Dragon Blooded artisans can maintain most surviving First Age tech, but any technological advancement since the First Age can't compare to what Twilight Caste Solars were capable of.
GURPS has the idea of Technology Levels built into the rules, by name; societies have a tecch level, as does each character (representing the highest level of technology with which they're familiar). But as a generic system, it also recognizes the idea of Schizo Tech, handling it with mechanics such as Split Tech Levels (for societies which have advanced further in some areas than in others) and the Cutting-Edge Training perk (for characters who've received technologically advanced training in one specific area). Some examples of published GURPS settings with Schizo Tech:
The alternate historical timeline of "Alexander Athanatos" from GURPS Bio-Tech is mainly in the Iron Age, but it is capable of producing genetic hybrids thanks to Hippocrates triggering a revolution in medical science.
The world of Yrth, setting of GURPS Banestorm, is a vaguely-medieval fantasy world like many others, except that people from Earth occasionally get teleported there and stranded. The Powers That Be suppress gunpowder and other obviously problematic technologies, but many minor technologies and concepts have become common, including the germ theory of disease, some experiments in vaccination, heliocentric astronomy with elliptical orbits, the modern novel, stagecoaches with suspensions, sloops and brigs, fingerprinting, and the use of perspective in art. Advanced printing has become established despite the problems it causes.
The random alien culture generation rules in 3rd edition GURPS Space have a small (a roll of 3 on 3d6) chance of resulting in primitive barbarians with spaceships.
The Discworld Roleplaying Game uses a cut-down version of the full GURPS rules, but given that the Discworld is heavy on Schizo Tech along with depictions of technological progress, this necessarily includes the relevant tech level rules.
The Space 1889 RPG was all about this trope. Though most of the weapons described in it are either historically-accurate late 19th century weaponry or very rare Steam Punk inventions. Martians use rather primitive weapons but they are, all in all completely different civilization.
Traveller: There is a lot of space in, well, space and some stuff never gets to some planets. Also there have been a large number of disasters in the Traveller history. And even those from high tech cultures like to go retro on occasions, like using swords when they fight a Duel to the Death.
New Horizon was colonized by humans with advanced technology... and low resources. Thus, while every town has touches of modern inventions—a few computers, a Promethean or two, the ever present Wafans—the setting as a whole generally features more frontier-level technology, like flintlocks and rifles.
In Fading Suns most advanced technology is prohibited or restricted by the Church following the fall of the Second Republic, though it's not always enforced, particularly weapons tech. For example, a militia man on a backworld may have a laser, while his wife still cleans the shirts on the rocks by the stream.
The technology in The Splinter covers everything from early medieval weapons to impossibly advanced, essentially magical devices. The core rulebook includes repeating crossbows, monofillament razor-wire launchers, steam-punk Gatling guns, automatic shotguns, advanced underwater laser pistols, heavy insanity rays, blade-wands, disintegrator pistols, directional nukes, and about fifty types of old-fashioned medieval slaughtering tools.
All over the place in Rocket Age. The humans use conventional 1930s technology alongside spaceships and RAY guns and the Martians used to use a mixture of medieval and high tech technology and are currently in the desperate process of modernization. Then on Jupiter we have the use of zeppelins and compressed air dart rifles, to ensure that things don't go up in smoke.