In response to the 10 most puzzling ancient artifacts post, Bill Whitcomb says:

OOPARTS (out of place artifacts) happens to be one of my favorite areas of pointless speculation. Some of them, like the Greek calculator and the stone sphere are what they seem to be. That is, the Antikythera device was the product of someone brilliant, but was totally within the abilities of period craftsmen, providing someone could design it.

Others, such as the Dropa stones are more cryptic.

The problem with many of these reports, such as various things found in coal, is that they were found in the 19th century and were not documented in any verifiable way. Someone CLAIMS they saw the thing fall out of a lump of coal and, often as not, the artifact itself has been misplaced.

There’s tons of this stuff in the book, Forbidden Archeology:

Here are also a few related OOPART sites: