Cybernetics will only reach full viability when they can be tied into the human nervous system. Replacement sensory organs are obvious, but a cybernetic limb will not have the same fluidity and grace of a natural limb until the brain can treat it like one. Some method of translating between electrochemical and digital transmission is needed.
Neurons carry electric signals not unlike wires in a computer chip, so chips which read from and write to neurons can be (and have been) made. However, neural signals look like a mess of spikes◊ while wire signals tend to have simpler patterns◊. Translating between these types of signals is not easy. Fortunately neural systems can learn, so once this interface is in place, someone with a replacement cybernetic limb can practice with it until they can use the limb as well as the one they were born with.
Correspondingly, many attempts at power-multiplying exoskeletons have resulted in broken bones and strained muscles when the exoskeleton multiples the force and speed too far. There have been some recent successes as designers learn to work around this.
At the other end of the scale, there's a robotic arm which has been made deliberately weak. The fact that it is too weak to hurt anybody is a selling point.
Any cybernetic device that must enter the skin (as most fictional cybernetics do) will require a breach in the tissue that will become infected. People with shunts and similar implants constantly have to cover and medicate the opening, and routinely get infections despite their best efforts. Creating an airtight, infection-proof seal over the point where the implant enters the skin will be a necessary prerequisite for fully-integrated cybernetics.
Or eliminate the need to breach the skin, such as by transmitting data (and maybe power) via induction to and from something implanted just below the skin.
Poisonous creatures (such as monarch butterflies) sometimes form an immunity against their own poison.
Venomous creatures mostly aren't immune to their own venom though, so must isolate it from the rest of their body kind of like the same way we have hydrocholoric acid in our stomach (with a very complicated "support" system to neutralize it) but aren't by any means acid-proof, as anyone with acid reflux issues (a failure of that containment system) can attest.
Scorpions are immune to the venom of their own species, and often their entire genus.
Similarly, the immune system of any animal can't fight diseases unless it's exposed to the pathogens first. While a healthy immune system will readily take care of many pathogens, some require vaccines, which contain weakened or the protein signatures of the pathogen, in order for the immune system to build up a proper defense against it in case it does show up.
Koalas eat nothing but eucalyptus leaves, which are poisonous. To get around this, they build up an immunity to the toxins in the leaves as babies... by eating their mothers poop.
The human body has a limit on the severity of the G-forces it can withstand before going into blackouts, physical injury and eventually death. Until this physiological limitation can be overcome (probably by Inertial Dampening, as cyborgification has any number of moral-ethical issues, though achieving this would also create its own issues), having a Cool Plane that is agile enough to literally turn on a dime is worthless as no pilot would survive using its full potential. One possibility is to make it a UCAV, but if the military-industrial complex has any savviness about AIs in control of weapon systems... Even making it remote-piloted like the Reaper UAV wouldn't solve everything, as there's still the problem of pilot reflexes, which will be exacerbated by transmission lag. Less of a problem when carrying out airstrikes using Herd Hitting Attacks on an insurgent convoy, but a dogfight is a whole different ball game.
High tech sensors and reconnaissance systems are only effective when paired with a means to analyze and disseminate the information. Case in point, Britain's revolutionary Chain Home radar would have been unable to stop the German air attacks had it not been paired with the Dowding ground controlled interception network. In contrast, United States Army radar operators on the island of Hawaii detected and tracked the inbound attack on Pearl Harbor, but had no effective way to raise an alert.
While front line combat forces are usually the focus of military budgets and popular imagination, most military leaders will tell you that the most important factor in battlefield victory is the humble art of logistics. Troops can't fight without ammunition, food or water, vehicles can't move without fuel and spare parts. In World War 2, when an American infantryman expended his personal ammunition stock of 80 rounds, it took the combined efforts of 17 other men to make sure that when he reached back there would be more waiting.
While it has been well documented that falls from heights of less than a few meters can result in severe injuries, the Required Secondary Powers of tumbling, falling properly and weight distribution is the basis for the sport of parkour.
And parachuting. The parachute landing fall and rollover is the thing which is taught to all newbies before their first jump.
Wolff's Law claims that after injury, the human body repairs itself so that it is stronger than before. However, this has since been discredited. While exercise make you stronger, injury does not. Very often, it makes you permanently weaker. It was once thought that repeated microfractures which healed would end up making your bones stronger, but it does not actually make you stronger. This is less than surprising.
And with time and lot of training, horrible deformity, bone pain and cumulative disability. Yes, it's a charm.
Don't forget the ballerinas. They basically break the bones in their feet and have them heal in an unnatural position. One reason why you might not want to give one a foot rub; black toes aren't exactly attractive.
Strippers' feet are the same way. Years of wearing heels that are commonly very tight and made of plastic leave thick, painful callouses, scars from breaking in the shoes, and often force the fourth and fifth toes to curl in since the shoes are so narrow.
Many experienced skydivers wear ankle braces. Not only because spraining your ankle leaves it permanently weaker, but also because they have landed hard so many times they want to prevent any further damage. Many experienced skydivers suggest novices to begin to use ankle braces early in their careers before anything serious has even happened.
People who grow too tall can suffer chronic pain because bones, muscles and tendons don't develop sufficiently to deal with the attendant weight due to the Square/Cube Law. People above 7 feet can have real problems, as Andre the Giant could attest.note 7'5" was André René Roussimoff's kayfabe height, he actually topped out a 6'11" but his gigantism and massive daily beer consumption gave him the proportions of a much shorter man. This is especially true because most people who are so tall suffer from various forms of gigantism which are what cause them to achieve such size in the first place; these disorders do not cause proportional growth and are often caused by tumors or other severe physical maladies which can kill the person if they go untreated.
The brain averts Dizzycam by inducing temporary blindness that exists for a fraction of a second. In other words, your brain averts Dizzycam by using Jittercam.
Human beings in general, actually. Water and oxygen are very corrosive. Their reactivity (separately and together) are the basis for all earth life, because they make biological reactions possible. Our bodies have to be built from the ground up to be immune, so that we can breathe air and drink water. (Consider that salt water and air can eat through an iron bar in a few weeks, but are harmless to human flesh).
Indeed, the earliest identified mass extinction in Earth's history was probably caused by the increase in oxygen in atmosphere poisoning most of the existing organisms, which were most likely similar to the rare microorganisms today found only in oxygen free environments.
This is more of a consequence that iron is orders of magnitude more reactive to oxygen than carbon (what we're mostly based of). The real wonder is how our body is able to make use of certain elements by themselves. For example, the nervous system makes use of pure sodium and potassium positive ions. Those two elements by themselves are very reactive and will explode in water, which for humans, is 70% water.
Pure sodium and potassium metals are extremely reactive. However, their ions, which our bodies use, are mostly unreactive, as can be seen here and here.
Oxygen is still corrosive enough to organic molecules that it rivals radiation as a cause of DNA lesions, and between the two cause more DNA lesions in a typical human being each day than there are grains of sand in the world. The fact that our bodies regenerate from that routinely is quite amazing.
Bat echolocation is amazingly high-energy, loud enough that some species can use their calls to stun their insect prey. As the Make Me Wanna Shout page observes, their own calls should be loud enough to deafen them — but they have an autonomic reflex which uses the stapedius muscle to dampen the vibrations of the inner ear. Humans have the same muscle reflex but it is really only useful for dampening the sound of yourself chewing.
Cicadas have an extremely loud call. In fact, they would make themselves deaf if they didn't have built-in earplugs.
This trope is technically a different way of saying "it's not the weapon that's deadly, but the man who uses it". You can own a gun and know how to shoot it. But even if you have a full-auto weapon, while it may give you an edge against someone with a weaker or slower weapon, having little to no marksmanship skills is a definite handicap in the least.
The human body, and the bodies of most animals infact, are filled with all kinds of required secondary powers, and often what isn't was itself a secondary power until some point in the evolutionary line a creature found to use the function in it's own right. Everything from the bones being used as anchors to muscles, to the shape of the blood cells, to the material of your teeth having to be water proof to not be eroded down by your spit. Many forms of birth defects and most of the effects of old age come from the body's lack/degrading of the secondary systems.
It's often overlooked, but humans are surprisingly strong animals for their body mass. Human muscles are capable of some astounding feats of strength, but applying all this strength simultaneously can rip muscles, tear sinew away from bone, or even cause the bones to crack or break under the strain. For this reason, the human brain's Required Secondary Power is a self-installed mental block that lets a person use only about one-quarter of what they're really capable of. This mental block only turns off when it's a choice between "use all of your strength, or die".
Crocodilians have the strongest jaw muscles in the animal kingdom today. In order to withstand what may be as much as 5,000 pounds of force (!!!) crocs need skulls that can withstand as much pressure as steel, and that are able to redirect huge amounts of force. Tyrannosaurs, which had strongest jaws of any land animal, if not any animal period, had to sacrifice cranial kinesis for a more solid skull design. They also evolved blunter, less blade-like teeth that are frequently compared to bananas, and railroad spikes.
In addition, huge powerful bite muscles = small, weak muscles for jaw opening. It's very easy to hold a crocodile's mouth shut. If the choice is between that and letting it bite you, it's very advisable; what's not advisable is getting into a situation where "hold the crocodile's mouth shut" is the best option.
Evolution and natural selection is actually based around this. Bill Nye, during a debate, actually points out that the term "survival of the fittest" actually means literally what it says; whichever creatures have the required secondary powers to fit whatever environment will obviously live long enough to pass on it's genes while those that lack the necessary powers will die out.
The reason a lot of modern technology would be diminished in usefulness in a post-apocalyptic scenario without the backup our civilization's infrastructure. Have a computer in your survival shelter? You probably have no internet. What are you powering your terminal with anyway, a gas generator? Well gas only lasts so long before your reserves become useless. Oh, that hydroelectric dam is still generating electricity for buildings that no longer stand? Too bad the cataclysm resulted in a lot of downed power lines and not many people left who have the know-how or equipment to fix them. Fine, forget broadcasting your Apocalyptic Blog from your computer, just go for a nice drive through the corpse-littered countryside. Oh wait, the gas. And there's a crater where the repair shop used to be. ~sigh~ Let's just shoot some zombies from across the street. Better watch your expended ammo, because chances are there are no more factories making new rounds.
In the opposite direction, most forms of technological advancements require secondary technologies that might seem irrelevant on surface. The principles of steam engines and computers, for example, were not unknown to ancient Greeks and Romans. The technology necessary to allow them to become any more than amusing curios? Not available yet, though.
Engineers in the 21st Century were able to demonstrate that Victorian-era technology would have been able to build a Difference Engine, which would have provided a massive increase in precision to a wide range of mathematical fields. They also demonstrated that it would have been too expensive for Victorian-era society to build, since only two engines have so far been completed. Much of our technology isn't amazing because of how advanced it is, but because it's cheap enough for the average person to purchase and use regularly; otherwise, it's just an expensive toy.
Even if a Bulletproof Vest manages to stop a round, the energy will still transfer, with bruises and broken ribs as likely results. Research is being done into shear-thickening fluids that will spread out the energy, thus reducing the blunt force trauma that results. Thus, you can keep armour at the original mass but with greater protective value, or also have lighter armour with the same protective value.
It was possible in theory for the Industrial Revolution to have happened in an era aside from late 1700s-early 1800s England. Steam power was apparently invented by Heron of Alexandria, and the consumer base that formed the market existed in 1200s China. Yet it finally took effect because of the necessary conditions of a middle-class urbanized England with a big local and continental market (via early Colonialism) to generate investment in new technology, and a stable national government that runs on support of institutions that run efficiently and dynamically.
When the list of choices for lunch companion(s) include ANY person, alive or dead, note that conversation will be non-existant unless your companion(s) — including yourself — speak the same language. Including time-period dialect, local variations and slang, and accents.
Some agreed-upon cultural references will also help a lot, otherwise your available topics of conversation will rapidly dwindle into nothing.
Prior to and during World War I, the Royal Navy was obsessed with having the biggest, baddest guns to hammer the Germans with. Unfortunately, due to the laser-like focus on the guns themselves, they never gave much thought to the actual shells they'd be firing. The result was British battleships firing huge projectiles at the enemy... which then due to faulty fuse design often failed to penetrate before detonating, if they detonated at all note Armor-piercing shells are supposed to do just that, pierce the armor of the target and then explode in their tender innards. . At the Battle of Jutland, the largest naval battle of the war, even German battlecruisers were hit repeatedly and stayed afloat (their British counterparts, meanwhile, promptly exploded). The ships that the Royal Navy managed to sink before this little snafu was corrected (not until the end of the war) were essentially pounded into scrap by kinetic force alone.
Firearms. A gun—and its internal mechanisms in particular—needs to be made of a material (metal usually) that can consistently handle the stresses of firing off live rounds. For this reason the infamous 3-d printed (plastic) Liberator Pistol is, at best, good for one shot before catastrophic failure.
Guns are also sophisticated machines with precisely tuned, often small parts. Even weapons lauded for their durability and idiot-proof qualities like an AK-47 are still full of springs, levers and precisely-shaped metal parts that have to fit well enough that the whole thing won't fail to fire, jam, or otherwise not work. To say nothing of the core component of any firearm, the barrel and chamber. If they aren't made to precisely hold the bullet so that the hammer can hit the primer, or if the bullet is allowed to bounce around inside the barrel, the gun simply won't operate as intended. Which is why, even with the proper machining supplies and tools, making a firearm from scratch is a lot harder than people think, and often results crude and unsafe weaponry.
The building blocks for rudimentary Powered Armor already exist, but without very high-capacity batteries or an engine or reactor small, safe and light enough to be man-portable, no exoskeleton is going to go far from base, if at all.