The easy-to-build device allows anyone to view any point in the past "live" - that is, as if watching a video recording. Imagine the ability to look back on the ancient civilizations and discover if Stonehenge really was built by aliens!
And then, someone discovers that they can use it to watch anyone, anywhere in the world, up to and including the present... and 100% of all privacy everywhere goes out the window. Now, that may not sound like Armageddon, but it would have a huge, drastic, and immediate impact on civilization as we know it. Imagine if an already trigger-happy government overhears another government talking about its destruction. Instant pre-retaliation! Nukes fly! And, the second they launch, everyone else knows about them too; thermonuclear war ensues, and the population is wiped out.
If the device came from a culture with no sense of privacy, they would never even suspect that it could be an issue.
Like the internet, only for thoughts! Imagine the ability to speak to anyone in the world through nothing but shared thoughts. The military would snap it up immediately, as it would create an amazing instant communication for soldiers in the battlefield. Eventually, telecommunications would be replaced with braincalls; on the one side, users can think of a recipient, and speak directly into their mind. On the other, senders can "blanket send," and anyone who wants can join in and listen.
And, just like the Internet, in a few years 90% of every message sent is spam. Unairconditioned rooms full of sub-minimum-wage employees sending think-a-grams to every person in the world. Productivity declines rapidly as people everywhere are overwhelmed by the massive amount of messages their poor brains are bombarded with; entire industries fall due to the inability of their workers to focus on their job. Imagine being an engineer in a nuclear power plant, but on the phone 24 hours a day, even being woken up at night, just to be asked if you want to buy printer cartridges. You are not going to do your job well; bye-bye power. And without power, there will be no food, no fresh water, no technology. Billions will die. That doesn't even consider those who just snap and kill themselves, or the others who snap and kill those around them...
Of course, the hive-mind that designed it loves the concept, and are confused as to why no one else likes it.
Everything You Could Ever Want
The machine reads the mind of whoever is nearby, and presents them with the object they most desire, in its best form. Hungry? It gives a filling, yet healthy meal. Have to use the bathroom? It builds a luxurious lavatory fit for an emperor. Too many people who want to try it out? It builds a copy of itself all over the world, where anyone can access it.
Of course, to power its creations, it uses the equivalent of five years of life. Whatever you wish for steals half a decade of your life away. Even without realizing it, humanity dooms themselves through greed. Their final, collective wish? "Life! I'm too young to die!"
"Huh," the machine replies. "That's new. I don't know how to do that."
The ancient civilization that built the machine was used to living for millions of years; as they aged, they realized it was better to experience life than to live nigh-on forever, and built the machine to create an Eden for anyone who did not wish to live forever.
Everyone loves toast. And everyone knows that the absolutely best toast is actually pulled from an alternate universe through tiny wormholes in the fabric of space-time. You didn't know that? It's true! Not many people have it figured out, but those "in the know" created the perfect toast-making machine. Using its own internal power source, each self-replicating toaster can produce up to one slice of toast per week! Warning: toast is actually summoned from an alternate universe.
All goes well, until someone has the bright idea of creating not one piece a week, but three pieces for each meal. To do that, he would need 21 toasters. But what if he invited friends over? They would want toast, too. So he creates 50 toasters, just to be safe. In fact, everyone in the world manages to get their hands on a few of these. The food shortage is solved with the advent of 100 billion toasters (some people really like toast). Every week on the dot, each toaster pops out a single slice of toast, weighing exactly 50 grams. 100,000,000,000 slices of toast every week, or 5.2 * 10^12 pieces a year.
On average, the planet gains about 40,000,000 kg a year due to dust. However, it now gains over 260,000,000,000 kg in a single a year! Worse, the sheer number of toasters - also built from wormhole material - begin making a rather nasty whining noise. As it turns out, all those wormholes together make one fantastically huge wormhole, destroying the entire earth.
The aliens who created the technology are flabbergasted. I mean, toast is great and all, but come on! It's just toast! Who likes toast THAT much? They never even considered the fact that anyone would want more than a slice or two a month.