Many people expect that along with aliens coming and visiting Earth, they will reveal secret technology to us. Especially free-energy.
But, what if like Glinda Good Witch, the aliens tell us that we have had continuous, non-polluting energy all along?
Electro-gravitics is one that 
Or Tesla's notes and writing.
Or Brown's theories
Or Kozyrev
Or Otis Carr
Even Maxwell's equations, if you can find them (they were changed, even in his own book, passed the 4th reprinting).
If aliens visiting us actually wanted to help us comprehend free-energy, would they give us new information, or would they have to start at getting rid all the things we think we know that we don't?
Being told that we know nothing about energy
Modern Materialistic Science is so proud of its Laws of Thermodynamics. Too bad that they only sorta work in a laboratory, on Earth, where everyone in the Lab also believes in them.
The reality is that the universe is created trillions of time a second. (or displayed, as on a video screen) This is the way we get/see movement. This also means that there does not need to be consistency in energy levels between frames. The universe can break the laws any time it feels like it. (Like, when you hear of a woman lifting a car off her child)
We have tons of over-unity devices. We just haven't made any of them into a continuous cycle with enough extra energy to power other things.
We already have perpetual motion machines, it is just that people "Believe the science" which says there can be no perpetual motion machines. So, do you believe your own lying eyes, or "science"?
And, if you asked an alien how to make a perpetual motion machine, don't be surprised if he pulls out a fridge magnet and a spring and shows you how to make, what is from his world, a child's toy.
Being reminded of the aether
We used to have all kinds of theories about aether
If space is a vacuum, then what is light doing there?
But, Modern Materialistic Science duped everyone into believing there was no aether.
Michelson-Morley experiment was barely an experiment, and if true, only disproved ONE model of the aether. Worse, they found something and called it nothing!
If we continued studying the aether, we would understand energy a lot more. We wouldn't have this silly notion of wave/particle duality. We might even know about teleportation. And we would certainly understand how gravity works.
Just wait, the aliens will tell us that Einstein is a fuzzy haired crackpot. (where have i heard that before?)
What if math is wrong?
One of the things scientific thinkers are proud about is math. Most believe that math is proven, and completely logical. Unfortunately, it is not.
What if π = 4, instead of π=3.141529… ?
The above image is a joke, but unfortunately, jokes usually have a lot of truth in them. It is really our viewpoint that decided if it was 4 or 3.1415. And, in a little while, when the aliens show up, and start talking with us, we will find that 3.1415 is π only on earth, or only in this solar system, or only near a star, and not near a "black hole" (which may not exist).
Imagine trying to talk to an alien to help solve some of math's problem, and finding math is not the universal language you though it was.
And, how would they not? Anyone who has looked into it knows that math has all kinds of internal inconsistencies. We do not handle ∞ (infinity) well at all. With it, we can prove that 1 = -1. We really need to work on that.
Finding out about the study of the obvious.
Imagine some of our most brilliant and revered minds have worked out amazing things, that aliens put into the basket, "the study of the obvious"
It is not that we are that backwards, it is more that we tend to over complicate things. Like anthropology being so against Intelligent Design. It is perfectly obvious that there is something more going on than random chance. But, we can't admit that to ourselves.
Soooo, we might learn about free-energy from an alien child, who shows us his neat toy. While the Modern Materialistic Scientist goes round and round trying to defend what he thinks he knows, as a fact, as a "LAW" of nature.
And really, the aliens might just point out, you have known all along.