Out-of-place artifact

An out-of-place artifact or OOPart is a name given by creationists and woo-pushing fringe scientists for historical and archeological artifacts they feel could not have been created by a particular culture due to a lack of knowledge or materials.

Creationists often argue that such "anomalies" show that mainstream scientific chronologies and models of human evolution are all wrong, and all this mysterious stuff is proof that Noah really built that Ark. Ancient astronaut fans maintain it's ironclad evidence of alien visitors' influence on mankind. Pseudoarcheologists are convinced it's proof that advanced civilizations existed during the Pleistocene Ice Age.

The idea of a "smoking gun" with the potential to completely overturn the scientific orthodoxy that's shunned them is a daydream that appeals to a wide variety of cranks.

Examples
Supporters such as Zecharia Sitchin claim OOPArts are evidence that vanished civilizations possessed knowledge or technology far more advanced than our own, alien beings undoubtedly visited the earth millions of years ago, and mainstream science is overlooking huge areas of knowledge. Adjectives like "mysterious," "unexplainable," and "anomalous" are liberally used to describe stuff like:

Carved stones (generally fakes or misinterpretations)

 * The Dendera lamp: Relief decoration within the Hathor Temple at the Dendera complex in Egypt. Snakes depicted inside elongated cocoons look somewhat like the filaments inside light bulbs, so ancient astronaut theories incorporate the carvings as evidence that the ancient world harnessed electricity for lighting.
 * The Ica stones: Decorated stones from Peru depicting everything from dinosaurs cohabiting with man to advanced medical procedures. A hoaxer owned up and showed how he'd done it. A good example of all the various woo camps using undatable objects from no valid archaeological context and calling it "proof" of whatever they happen to believe in.
 * The Los Lunas Decalogue Stone: Supposedly ‘proof’ that ancient Hebrew-speakers roamed the Americas — or at least New Mexico. Due to the unusual alignment of the Hebrew, more likely to be a modern hoax, or at least the work of a 16th century Spanish Jewish "converso".
 * The Tucson artifacts: Latin- and Hebrew-inscribed lead objects self-dating to the first millennium CE, apparently sealed in rock that's very much older, indicating the presence of a lost Roman colony in Arizona pre-Columbus. The lead they're made from is consistent with recycled car battery anodes, so these are likely a hoax. The facts that there's a dinosaur on one and another is a sword made from lead doesn't help.

Misplaced objects (often cases of drastic mis-dating)

 * The Wedge of Aiud: A wedge made from aluminum found in 35 feet of sand in Romania, said to be 11,000 years old. But it's simply a tooth from an excavator machine that fell off when construction workers were digging the hole.
 * The Coso artifact: A supposedly unknown electrical device encased in a 500,000-year-old geode. Actually just a concretion of iron around a rusting 1920's spark plug.
 * The London Hammer: A hammer partially encrusted with limy rock concretion found in London, Texas. Sometimes exaggerated as having been found "embedded in solid rock", it's merely the result of dissolved sediment hardening around a hammer.
 * The Dorchester Vase or Dorchester Pot: found at Meeting House Hill, Dorchester, Massachusetts, in 1852 following a mining explosion, which supposedly originated in Ediacaran rock. It isn't clear how its original location could be tracked with total accuracy from its place in the subsequent debris, and most mainstream experts believe it was an obviously Victorian artifact probably made just a few years before that became mixed in the rubble, likely a pipe holder.
 * The Geraldton Plate: Bronze plate bearing Egyptian hieroglyphic designs found in 1963, in Geraldton, Western Australia, in "a deep excavation below the current sea level". Cited by some as proof of ancient visitors to Australia. Actually an early 20th century Egyptian souvenir likely brought back by a veteran of the First World War.

Objects in coal and other geology (may be natural or left by miners)

 * The Wolfsegg Iron: An iron cube found buried in a 20 million-year-old coal seam, misleadingly described as a "perfect cube" when, in fact, it could be generously described as a lump with a few straight-ish edges.
 * Upshur Bell: an elaborate and clearly artificial bell in brass or similar alloy, supposedly found in coal by a ten-year-old boy. Creationists say it was found in 300 million-year-old coal and contains a highly unusual alloy. There is reportedly an ancient Babylonian demon on top of the bell. Unfortunately, its discoverer, Newton Anderson, failed to save any of the coal, either attached to the bell or otherwise. It does not seem to have been seriously studied by skeptics, but plausible hypotheses include it being, like the Coso artifact or Dorchester Vase already mentioned, an item of much later origin which became mixed with the coal, or, like the Tucson artifacts, simply an outright hoax.
 * Iron Pot from Sulphur City: an iron pot reportedly found in coal about to be burnt in Sulphur City, Oklahoma, another of several artifacts purportedly found in coal. It appears a similar case to the Wolfsegg Iron. Metallic-appearing pyrite is commonly found embedded in coal, as is metal from mining machinery.
 * The Klerksdorp spheres: Geological curiosities found in 3-billion-year-old pyrophyllite deposits, often misleadingly described as "perfect spheres" while actually being far from perfectly spherical.

Plant and animal (not actually man-made at all)

 * The Baigong Pipes: Rusty, hollow, metallic pipes in the caves of Mt. Baigong in China's Qinghai Province ranging from needle-thick to over a foot in diameter imply some sort of ancient municipal water system made from alien metallurgical techniques. Actually just fossilized tree roots.
 * The Eltanin Antenna: A picture taken from a boat of an antenna-like object seemingly staked into the seabed in the deep ocean. Later recognized as , a sponge.
 * The Paluxy River ‘footprints’: Claims of footprints of dinosaurs and humans preserved together in one stratum of rocky riverbed, supposedly made while running from the rising waters of Noah's Great Flood, except the tracks are not human footprints; they were made by the same three-toed dinosaurs but with only the middle toe visible, some of which have been modified to make them look more human.
 * The Starchild skull: Supposed evidence of an alien-human hybrid child. Really just the skull of a child with a congenital deformity.

Regular ancient artifacts (old things that don't require any further explanation)

 * The Diquis spheres: Stone spheres of varying sizes from Costa Rica, some over 2 meters in diameter. They're not perfectly spherical as claimed by woo sources, but are impressively close. Scientists believe the spheres were man-made hundreds or possibly thousands of years ago, though their purpose is unknown, and are attributed to the lost
 * The Iron pillar of Delhi: Said to have been constructed in 912 BCE of a highly advanced iron-based alloy that cannot rust. But wait, it has some rust near its base, and it more likely has its origin in the reign of Chandragupta II, 380-c.415 CE.

The Antikythera gambit
The Antikythera mechanism is a mechanical computing device dated to the late second or early first century BCE that was found in an ancient Greek shipwreck. It is often used by fringe science promoters to argue that stuffy old mainstream science should never dismiss fantastic claims of OOParts, because scientists' original conclusions about it were wrong.

In reality, devices like the Antikythera mechanism are mentioned in multiple ancient Greek and Roman historical sources written by authors such as the Roman orator Cicero, the Greek mathematician Pappos of Alexandria, the Christian apologist Lactantius, the Roman poet Claudian, and the Greek Neoplatonic philosopher Proklos the Successor. Cicero in particular gives a detailed description of such a device in his De Re Publica.

Further undermining credibility is the general misunderstanding of the Antikythera mechanism as a "computer": the mechanism itself, while extraordinarily advanced for its time, is incredibly primitive by modern standards — it is not Turing complete and, thus, would be utterly incapable of running any program beyond the one for which it was specifically designed. Indeed, it is even questionable to call it a "computer" because of this, and the more research is done about its design, the less outrageous it becomes to envision the chance that even ancient Greeks — who were known for their fascination with machinery — would have designed something like it.