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.
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.
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.
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 Genre Savviness about AIs in control of weapon systems...
Or just have the best of both and make it remote-piloted like the Reaper UAV.
Even then, there's still the problem of pilot reflexes, which are likely to be worse with a remotely-controlled interface than in a cockpit.
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.
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.
Athletic or physical training in general. It is generally known that one of the most sure-fire methods to slowing down aging is consistent physical and athletic activity. Such activity causes the body to draw a greater number of nutrients from food, forces the body to produce regenerative hormones, and even forces the brain to maintain higher cognitive abilities into old age (aka assists in preventing Alzheimer's) due to the fact that a body constantly in physical motion requires the nervous system to maintain constant contact with all the assorted muscles, tendons, and body parts et al.
However, constantly putting your body under great physical stress only wears it down immensely. The metabolism of all the cells is in overdrive to provide the energy needed for all that physical activity. The cells don't get the proper time needed to rest and regenerate. As a result, life expectancy drops. Exercise, but don't overdo it. This all also applies with overeating. With all the nutrients provided from the food, the cells are constantly burning up energy. Digestion itself costs energy as well. Once again the cells don't get the time to regenerate properly. Life expectancy drops. Humans were designed to eat only a bit every day or two/three because of the scarcity of food during the hunting days. Our current lifestyle makes us eat too much food a day and wear down our bodies too much. That's why people who have suffered from famine have a lower chance of heart failure than people who eat three meals a day. There are currently diets that revolve around periods of feasting and periods of fasting, to give the body enough nutrients but also enough time to regenerate, so that life expectancy increases.
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 giant if they go untreated.
One problem in the developed world is that, while advanced agriculture provides abundant food, our metabolic systems are still not that dissimilar from Stone Age hunter-gathers. This often leads to calorie intake outstripping energy needs, causing weight gain. It is not increased food consumption but decreases in physical activity that is responsible for this change.
The mutation that broke our ability to produce our own vitamin C became fixed in the population because we were getting enough in our diet that our disability didn't matter at the time.
The high, year round calorie intake and lack of physical activity is why Type II Diabetes, once rare and found only in the middle-aged, is becoming very common and is appearing even in children.
Although, in some cases, this has been the case. The genetic mutations for lactose tolerance and for the ability to digest massive amounts of proteins and fats have been historically shown to have changed thousands of years after the dietary change.
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).
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.
Not that trying to is advisable.note Actually, if the choice is between that and letting it bite you, it's very advisable; what's not advisable is getting into a situation were "hold the crocodile's mouth shut" is the best option.
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.
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.