The Antikythera mechanism is usually filed under "interesting anomaly" and left there. A clockwork computer from 100 BCE that models planetary motion, eclipse cycles, and the Olympic calendar with a precision that Europe wouldn't match until the 14th century. The standard narrative absorbs it as a one-off — a genius in a workshop, ahead of his time.
But "ahead of his time" is doing a lot of heavy lifting. Gear trains of this complexity don't emerge from nothing. They imply a tradition, a lineage of craft and theory that we have no other evidence for. Either that tradition existed and was lost completely, or our model of ancient technological development has a hole in it large enough to sail a trireme through.
I'm not suggesting aliens. I'm suggesting we're comfortable with gaps in the record that should keep us up at night.