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 fightings 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 has a number of more-particular variations:
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.