The printing press. Metal type would have been a stretch, but fired-clay would have been a snap.
The Chinese had presses using hand-carved wooden masters long before Gutenberg. The first book is circa 750
Of course, it’s not movable type, which makes all the difference
According to the Wiki article, they had metal moveable type by the 12th century (!)
The Chinese also used wood (not metal), and did not have an alphabet. Metal was key because printing using blocks isn’t a huge improvement over hand copying.
The Romans had windmills. IIRC, there is actually a quote of an emperor saying, “But how will I employ my people?” The windmill probably required a certain level of depopulation and political chaos.
There are similar problems with most of the suggestions above, with the exception of anal beads. So, I’m going to suggest the bikini.
One thing I learned from an instructor when I went back to college a couple of years ago, is that there is a lot of technology we have now that we could have had years, or even decades ago, but the companies who came up with them didn’t put them out sooner was because they couldn’t figure out how to make a profit off of it.
Unfortunately I can’t think of any examples.
What you cite from my post already says this. It also says that they had metal before Gutenberg.
I’m sorry, I should have been more explicit, and not everything I said was right. I just finished listening to “The Late Middle Ages” by Philip Daileader of William and Mary. Obviously, he could be wrong, but I trust him more than Wiki in this area. In lecture 16, he states that:
- The Chinese were using *movable * wooden and ceramic type by the 11th century.
- The Koreans were using movable metal type by the 13th century.
What the Asians appear to have lacked was a press mechanism, not movable, metal type. And, of course, a simple alphabet.
Yokahama bath houses had a specialite de la maison where the girl would work a length of soapy knots up your chute.
I seem to recall a Sci-Fi (Definitely not SF!) story about a Confederate-sympathizing scientist who used a time machine to deliver a very simple sub-machine gun (possibly a Sten gun) to the Southern side in the Civil War. He claimed that the design and manufacture of the gun was well within reach of the technology of the day, once they had a prototype.
No idea if this claim would have been true.
Passive-solar housing could have been built as early as the late sixteenth century; the most critical component is plentiful glass windows. Passive-solar also requires thermal mass to store the heat and insulation to keep it from escaping. You also need a vapour barrier on the warm side of the insulation to keep water from migrating into the insulation and condensing.
Thermal mass can come from stone, brick, or rammed-earth walls; insulation can come from straw or cob. The vapour barrier can be made of pitch. All have been used for millennia.
Hardwick Hall (the 1590s) had walls of glass windows. But glass was a thing of the rich then, and anyone that rich dould afford the fuel to heat any old pile of stone. So passive-solar never occurred to them then.
This article is relevent to this topic!
Harry Harrison’s Rebel in Time; it was actually a racist Army Colonel with access to a secret experimental time machine. He took a Sten gun and a set of plans with him, along with enough gold to finance his plans.
I have always wondered if there was anything really stopping the Romans or ancient Chinese from developing rudimentary Bessemer process. That would have changed history a bit.
And it took politicians to really utilize the screw.
As we all know, politics wasn’t invented until the Policean Council of 1714. 
Inoculations. The Chinese were doing it in some form for centuries, and their practice was even mentioned in Western writings from time to time but treated as some sort of oddity. Then finally it caught on in the West with the smallpox vaccination.
The Minié ball, or one of it’s contemporary cousins. Basically, a conical piece of lead with an indentation in the base that allows you to load a rifled musket as quickly as a smoothbore musket. Quadruples the range at which you can hit a man-sized target. There’s no reason this couldn’t have been invented as far back as the matchlock arquebus circa 1500 AD, and it would have changed over three hundred years of military history. But in our timeline it was only invented about 15-20 years before machine tooled breechloaders and the brass cartridge made muskets obsolete.
Band name!
An all-time classic of this genre – wheeled luggage. How long have people had wheels? How long have people had luggage?
I blame religion.
Certainly could have been – in fact, it was done.
The Gatling gun was patented in 1862, and actually demonstrated for the Union Army – in real combat, no less. It could achieve a rate of fire of up to 200 rounds per minute, about the same as a whole company of soldiers.
It was rejected by the Army bureaucrats because it was ‘too wasteful of ammunition’.
Are you serious? So that’s why the Boer war was its first practical debut? I never knew that.
Hinged cell phones that put all their buttons on the inside so you don’t make butt calls. The technology to do that is available now, and even 10 years ago and more. But Verizon insists it’s never been done.