The general study of retro-technology strikes me as the premier field for demonstrating that technical knowledge is not a matter of linear cumulation over time, and one of the best illustrations that the past truly is a foreign country. There’s nothing further from the truth than A Connecticut Yankee in King Arthur’s Court : cast into medieval Europe, or 12th Century Africa, or Ming China, and magically granted perfect ability to speak and understand languages, a modern person would still flounder simply in their ability to use, not to mention make, the characteristic technologies of the day, both everyday and extraordinary ones.
The Secrets of Lost Empires series on the PBS program Nova did a marvelous job of demonstrating that a few years ago.
Turns out it’s pretty difficult to build an effective fortress-smashing catapult. Merely having an expertise in modern engineering doesn’t allow you to just quickly McGuyver one up out of a few logs and some rubber bands.
It's great stuff, at any rate. I think one of the dream research projects in the whole world would be putting an inventive technologist together with a historian, giving them some millions of dollars and institutional support, and letting them methodically study retro-technology. Not only is it a great cure for the hubris of modernity, it’s a pretty instructive guide to the kinds of actual social and economic infrastructure that past societies must have had to build and use their characteristic technologies.