CHECK IT OUT ...

Classical historians may be able to help Eugene Volokh. He's looking for

items (products or processes) that satisfy all these criteria:
They were unknown to people in ancient Rome circa 150 B.C.
They could be manufactured with then-existing technology and then-available raw materials.
They would be at least modestly useful in that era.
Even a nontechnically minded person today -- say, a smart 12-year-old -- would know how to make and use them. This is particularly important, and one on which many suggestions seem to founder.
Their absence would be pretty clearly visible.

"Stirrups, whipped cream, cowpox as a vaccine for smallpox, penicillin, Arabic numerals, the abacus, sterile technique, distillation, the printing press, the scientific method, pasteurization, the horseshoe, the toothbrush, the compass, the wheelbarrow, glass lenses, gunpowder, soap, and horse plow collars" have been commonly suggested, but some of them don't meet all the criteria. The abacus is out because the Romans had it.

Sasha Volokh suggests that you have a look at Qveere Eye for Thye Medieval Man. And the 21st century guys thought they had full time work on their hands!

If you don't mind the spoilers, Cliopatria's Tim Burke has a critically appreciative review of"The Return of the King" at Easily Distracted.

Twenty-five years after I invented the A-bomb .... Well, ah, it wasn't exactly me who did it and he didn't actually invent it, but John Aristotle Phillips got an A on his Princeton term paper for his figuring out how to make one and life's been downhill ever since. You end up indiscriminately being a fund-raiser for Bush, Hillary Clinton, Trent Lott, and Joe Lieberman.