The Coso artifact, dug from a California hillside in 1961 and claimed to be prehistoric, was "actually a 1920s spark plug that had become encased in a concretion." It belongs to the category of the out-of-place artifact, or OOPArt — an object found where it seems to "challenge conventional historical chronology by its presence in that context," as Wikipedia puts it. The Babylonokia, a "clay tablet shaped like a mobile phone and created as an artwork in 2012," later circulated online as an 800-year-old archaeological find.
A few genuinely puzzle archaeologists. The Antikythera mechanism is a mechanical computer built between 150 and 100 BCE whose "design and workmanship reflect a previously unknown degree of sophistication and engineering." The Maine penny is an 11th-century Norwegian coin found in a Native American shell midden: "Of the nearly 20,000 objects found over a 15-year period at the Goddard Site, the coin was the sole non-native artifact."
From Wikipedia: "most purported OOPArts which are not hoaxes are the result of mistaken interpretation and wishful thinking." The Klerksdorp spheres are not perfect spheres, the Wolfsegg Iron is not a perfect cube, and the famously "rust-proof" Iron pillar of Delhi "has some rust near its base."
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