There are some technical ideas that could have been developed much earlier, changing the ancient or medieval world.
A sterling example is the hot-air balloon. It was actually invented at the close of the 18th century, by a person who observed laundry billow up when dried over a fire. That same observation could have been made millennia earlier. There is no technical or knowledge barrier that made this invention impossible in ancient Greece; in fact Hero of Alexandria had knowledge of pneumatics far in excess of what would have been required, and tight lightweight fabrics, the only high-tech material needed, were readily available (albeit expensive).
Another, less flashy example is the star fort, or Vauban fortification, where walls are angled to be enfiladed from bastions, and bastions are angled to be enfiladed from the walls. This development was key in the era of the cannon; but it would have been quite as useful in the era of the ballista or longbow, totally obsolescing machicolations, and required no specialized technology or knowledge. It could have been developed in the Roman era, or in the era of the Crusader castles. But it wasn't.
My question is, are there other inventions that could, without anachronistic scientific insight or technology, have been invented much earlier? But that have been missed or delayed?
Some additional conditions:
- I want to specifically exclude the concept of gunpowder. Alternate histories based on its earlier invention have been done to death already. The same goes for DaVinci's (unworkable, BTW) vehicular concepts.
- I am asking about breakthrough ideas, such as inventing the sail, not incremental ones, such as building bigger ships with more sophisticated sail arrangements.
- My question is specifically about technical inventions, not geographic or political developments (Viking colonization of America) or social and religious concepts (women's suffrage in the Roman republic).
- I am thinking about a medieval world being revolutionized by an invention, but antiquity is OK too; not the modern era, though. Dirigibles in the Napoleonic Wars are an interesting idea, but outside of my scope of interest.
- Finally, I would prefer ideas with a military application, but this is not a firm requirement.
EDIT: I am required to "Update the question so it focuses on one problem only." As this would effectively require me to name the specific invention I am looking for, I cannot reasonably comply with this request. I would like to thank all contributors so far, especially the insightful individual who came up with this demand. I consider the question answered.