My favorite example is the Antikythera mechanism. It is quite clear that this had not been a one-of-a-kind, but that there must have been (many) predecessors of this technology to arrive at the kind of workmanship and layout that doesn't show any marks of hesitation or corrections as in the sole example, we know of. And that we know of this example at all, is just by sheer luck.

Generally, we only know of landmark size applications of historical technology, which tend to be rare.