Anachronism Stew

"Spare me your space-age technobabble, Attila the Hun!"
Zapp Brannigan, Futurama

Stories often confuse two or more time periods. For example, Renaissance dress may appear with 12th-century crusaders in a story set in Charlemagne's empire. Imperial Roman troops might have guns. Cavemen might be fighting Dinosaurs to survive. This is not only a modern trope. Medieval artists, for example, routinely dressed Biblical figures in contemporary fashions, and the Greek myth of Theseus features similar confusion. Some critics think the very first writer to actually try to reconstruct past times as different from the current era was Sir Walter Scott.

This can also be done deliberately. A work might combine large numbers of anachronisms to create a timeless or surreal setting, where the exact era of the story (or the era the fictional world is supposed to be imitating) is both ambiguous and irrelevant, allowing the work's creators to freely introduce further anachronisms whenever doing so would make for a good plot or a good joke.

This has a number of more-particular variations:

Compare Artistic License History, where the historical inaccuracy is the result of fictionalization, not a combination of actual historical details from different eras.

Please note that this is not a place to pothole any anachronism you find in a work. Those examples belong on the Trivia subpage of that work. This Trope is about the setting/environment of the work, and as such, requires multiple anachronisms affecting how the viewer of the work sees the setting.