Adults Are Useless: Truth in Television to the extent that all humans are by nature biased and fallible, and society gives adults power over children. But children rarely have the knowledge or experience required to deal with dangerous or otherwise critical situations, and this trope can lead to them not getting help when they need it in such a situation.
All Animals Are Domesticated: No, pandas are not "cute and cuddly" and will maul your ass like any bear would if you get too close. Same for many other animals - even some "domesticated" animals can be dangerous if approached in the wrong way, and wild or stray animals should be left alone.
Almost Lethal Weapons: It's extremely hard to inflict minor wounds on someone with a lethal weapon. Even "nonlethal" weapons such as tasers, tear gas/pepper spray, and bean bag shot can be lethal if used improperly or under the wrong conditions. Tasers specifically can kill with a shot to or nearby the heart, just like any electric shock to the heart.
Annoying Arrows: Don't yank that arrow out, unless you like torn organs and muscles, infections, and/or bleeding to death. In fact, try not to get shot by arrows at all. There's a reason why crossbowmen and archers were hated in medieval Europe. Also, arrows are even more likely to cause infection than bullets because bacteria on an arrowhead or arrow's shaft isn't killed in the process of firing.
Appeal to Nature: A form of the equivocation fallacy—conflating "happens on its own" natural with "consonant to a thing's nature, proper, fitting" natural. A lot of things that happen on their own are not consonant to the nature of the things they happen to (mostly because a thing that's been destroyed doesn't even have its nature anymore).
As Lethal as It Needs to Be: Weapons in Real Life are generally biased toward lethality, and the skills needed to not kill someone with a generally lethal weapon are beyond most people, especially in the heat of the moment. Guns are never this trope in real life.
Ass Shove: Shoving things up your ass at high speed is not safe for your anus or rectum, and many objects or substances are just plain dangerous to put up there.
Ballistic Discount: In real life, robbing a gun store (or even looking like you are going to, by not heeding the shop's rules for either keeping a firearm concealed or open and unusable) is pretty much equivalent to a suicide attempt. At least one person tried, winning a Darwin Award for his effort.
Barehanded Blade Block: Unless you are a super-sonic ninja, please don't try this with any sort of blade. Dodging is better for your hand.
Blind Driving: Not being able to see the road in front of you makes you a hazard to yourself, the people in your car, and everyone else on the road.
Booze Flamethrower: Slightly less dangerous than the Aerosol Flamethrower above, but don't do this unless you're a trained performer. If you don't blow hard enough, don't duck fast enough, or don't blow it out the right way, you are getting a mouth full of fire. Also, the tissues of the mouth absorb, so enough practice with booze will make you drunk - but using methanol at all will painfully kill you, and using gasoline or kerosene will irritate your mouth and throat and raise your risk of cancer of both, especially the more you do it. Also, accidentally inhaling flammable oils (even when not aflame) causes chemical pneumonia that is often fatal without immediate medical attention, and can cause severe chronic breathing difficulties even with it.
Brandishment Bluff: For the same reasons as Weapon for Intimidation below, pretending one has a weapon sometimes works in Real Life self-defense but can actually make things much worse - especially if one accidentally encounters or is reported to law enforcement or if the criminal is actually armed and recognizes you really aren't.
Bridal Carry: Unless there is no other alternative in carrying someone from danger, do not carry someone like this, especially if the accident/situation could have injured their neck or spine and/or if they have preexisting neck or spinal injuries - it will cause worse injury especially for someone with neck injuries because the neck is not supported at all, and the spine beyond the neck is only partially supported.
Bulletproof Human Shield: Human bodies are pliable and not great against supersonic metal. This is especially true for armor piercing rounds and high caliber.
Bulletproof Vest: Bulletproof vests stop some bullets and pretty much nothing else. Knives and arrows get through, and can slip past ceramic plates meant to guard against them. In addition, even the highest-rated vests are only meant to withstand a few rifle shots - and that's if you're lucky. Even if the armor does absorb the bullet, it is merely spreading out the impact, meaning that you'll still get a nasty bruise and can even break ribs. Armor tends to make people take greater risks, thinking themselves invincible, which isn't the case - cover is the best armor you've got.
But We Used a Condom: This trope often unfortunately convinces people that one of the best methods of pregnancy and STD prevention is useless or pointless - leading them not to use it at all, as the page says, putting themselves at far higher risk of an accidental pregnancy and/or of contracting HIV than the very low risk they would have if properly using a condom.
Chainsaw Good: Rule of Cool aside, chainsaws are not intended to be used as weapons and are as likely to kill you as the guy you're aiming at (kickback is one mean mother, especially more so on bones that are harder than wood) or break and jam. Anything harder than wood, like say...BONE, can cause the chain to break and go flying wild into YOU.
Choke Holds: Will most likely cause brain damage from asphyxiation, can kill, and can cause lasting damage to neck arteries or the cervical spine. You also need to be rather big and strong to pull this effectively. Don't Try This at Home.
Concealment Equals Cover: Don't assume that the wooden door, fence, interior wall, car body, or even concrete bricks you're behind will protect you from bullets. It won't.
Convection Schmonvection: Being near a fire, a lava pool, a hot oven, a nuclear or conventional explosion, or some other source of radiant heat can be just as deadly or injurious as being in it. A flashpoint is where an object instantly catches fire when the air is a certain temperature.
Cool Clear Water: Just because water is clear doesn't mean it is safe to drink. It may have lethal chemicals, organisms, or be lethally hot. In fact, the water may be clear DUE to those things: it could mean everything in there (besides microbes) is dead, usually indicating some amount of cyanide. This is especially true in areas like the high mountains in midwestern USA, where cyanide was used in gold mining. there are pools, ponds and even small lakes where there is no life at all due to the generous use of cyanide.
Cool Pet: Keeping a pet bear, tiger, cobra, ape, etc., is often an extreme strain for both owner and pet, and exceedingly dangerous.
Cure Your Gays: While thankfully no longer presented as valid in most mainstream media, this trope is still presented and marketed as valid in some conservative and religious media. The risks of these therapies can include severe psychological trauma, financial loss (paying for repeated therapy that does not "work"), depersonalization, dissociation, derealization, unhappy or unhealthy relationships, and suicide among others - as well as not encouraging safe same-sex behavior when the new "heterosexuals" "backslide."
Defiled Forever: Most definitely notTruth in Television in any of its variants. The belief in this trope often leads to widespread mistreatment of victims of domestic abuse, sexual abuse, or rape. There is nothing that makes someone who has been victimized by these crimes somehow different from any other human being, and in no way are they to blame for it. While some victims may have physical damage or suffer from PTSD, many do not, and those who do are not "crazy" or "dangerous" or "unsuitable." Nor does being a victim of child sex abuse make a person likely to become an adult offender committing the same crimes note There is some raised risk of an abused child acting out what he or she is "learning" on other children while the abuse is ongoing, but said "copycat" abuse almost always ends as soon as the child knows that the behavior is wrong/the abuse is ended, and abused children aren't more likely to become pedophiles as adults than anyone else.
Destruction Equals Off Switch: With almost anything involving unknown or unexploded chemicals or gases, destruction may render the device or situation more dangerous or unstable than just leaving it alone until people qualified in safe cleanup/shutoff procedures can arrive. Meth labs are notorious for this - if you ever see one, don't try to clean it up. Radioactive sources are often far more dangerous when destroyed or damaged, since the shielding on most is the best safeguard to a fatal release.
Do Not Touch the Funnel Cloud: The visible part of the tornado is merely the center. The tornado proper extends quite far away from the funnel cloud, to the point where you must distance yourself quite a bit from it if you don't want to subject yourself to winds sometimes in excess of 200 miles per hour. Beyond that you are still in danger of shrapnel. Thunderstorms severe enough to spawn tornadoes can also contain other lethal and injurious hazards, specifically lightning, large hail, and enough rain to cause flash floods.
Drinking Contest / Drinking Game: The more extreme of these have resulted in deaths from alcohol poisoning or injuries sustained while being so drunk one is unaware of oneself, and people becoming victims of rape or sexual assault while passed out or otherwise too drunk to consent.
Erotic Asphyxiation: Risky, for obvious reasons. Even partners or professional "workers" trained in such still carries a risk of, well, dying. Damage to arteries or the cervical spine are other risks that, while they might not kill you, may induce a (sometimes delayed) stroke or leave you with chronic pain or paralysis.
Every Car Is a Pinto: In real life, it is extremely rare for a car to explode following an accident, or even to catch fire at all. Because of this trope, people have refused to use seatbelts out of fear that they get stuck in an exploding car and injured persons have been dragged out of vehicles after accidents with no actual risk of fire, leading to severe injuries and fatalities that could have been avoided. The trope is truth for race cars and some supercars with modified fuel systems, but even then, wearing the seatbelt and safety gear keeps you conscious so you can use the fire extinguisher or escape before the fire gets to you.
Explosion Propulsion: Sure, explosions do make things fly quite fast... body parts included, or other people in the way of it. Or parts of the thing you're trying to make go faster. High explosives aren't toys, kids!
Faking the Dead: In real life, you could get arrested for this. The charges will vary though. note Simply lying down and pretending to be dead/setting up an even slightly convincing "fake crime scene" as a prank will probably get you disturbing the peace and/or making a false police report. A more complex attempt at it for financial reasons/starting over a new life will get you charged with everything from conspiracy to commit fraud to identity theft for your new identity at least.. The Playing Possum variant to attack someone will also lead to lots of criminal charges note Faking death in order to harm someone will turn an assault into aggravated assault, a killing to first-degree murder with lying in wait as a special circumstance, and doing it in the context of a war as a war crime. This should only be done if you're being confronted by bears or other deadly creatures, or if one is being violently attacked by another person and there is no other viable means of de-escalation or escape.
Farts on Fire: You can incur severe burns on your anus and genitals doing this if it goes wrong.
Finger In A Barrel: If you try this, it won't stop the bullet from destroying at least your finger. The shooter and his gun would be unharmed.
Fingertip Drug Analysis: Don't taste the mysterious substance if you don't want to get poisoned or burned by an acid or base.
Firing in the Air a Lot: What goes up must come down. Bullets fired into the air could potentially land on someone a fair distance away, injuring or killing them.
Friendly, Playful Dolphin: Dolphins are wild animals. They can be trained to be friendly and playful, but they are also one of the few species other than humans that engages in violence for sport (as opposed to hunting or self-defense), and some have killed or injured humans. Rape/forced sexual contact is also common among some dolphin pods.
Great Balls Of Fire: Pyrotechnic displays may make bands look flashy, but weak enforcement of fire safety codes has helped them cause nightclub fires that have killed hundreds.
Grievous Bottley Harm: Breaking a bottle almost always causes injury to the person trying to make a weapon out of it. Bottles used in TV are made of sugar glass, which breaks easily (and in a manner less dangerous to the one being hit by it). On the other hand, an unbroken bottle actually is an effective club... (Don't Try This at Home, kids!)
Gun Twirling: One of the few things you can do with a gun that violates every rule of gun safety, including treating guns as if they are loaded, knowing what could be hit other than the target, keeping one's finger off the trigger until aiming at the target, and never pointing a gun at anyone or anything without the intention to shoot. Attempts at doing this in real life have led to negligent discharge of the firearm(s) in question.
Hard Head: Any head injury should be treated immediately, regardless of whether the patient feels "fine". Brain hemorrhaging could be occurring. Note that the brain has no nerve endings to sense pain, only the membrane surrounding it.
Harmless Electrocution: Electrocution is lethal by definition. Electric shocks that don't accomplish death by electrocution (even "controlled" shocks such as electroconvulsive therapy, but especially lightning and high-voltage shocks such as those from tasers) can still create burns (including internal burns - just because an electric shock/lightning/taser victim only has small skin burn wounds does not mean that they have not suffered far worse burns to their muscles, bones, or internal organs. For example, a common injury pattern in lightning victims is silver dollar size burns on the back - but their spine and its nerves have second or third degree burns), heart arrhythmias, brain damage including ongoing seizures and memory loss, nerve damage, organ failure, and other disabling, possibly permanent damage.
Harmless Freezing: Hypothermia and frostbite are not harmless. Cells burst apart when thawed. Warm slowly.
Healing Herb: Some herbs are indeed medicinal. This means they have side effects, often unpleasant, sometimes lethal. And interactions with other herbs and with pharmaceuticals. And dosage requirements, which are a lot harder to judge in plants with natural variations than in carefully built pills, which means overdoses with all their attendant problems, or alternately, underdoses - not getting enough of the effective substance(s) for a proper therapeutic dose. Also, no herb is a panacea, and some that may be good for some problems could cause or worsen others, just like pharmaceuticals. In fact, a large number of common pharmaceuticals are based on the active chemicals found in herbs, artificially synthesized or purified to remove toxic chemicals found in the raw plant, and/or tweaked to remove unnecessary molecular structures which can cause side effects or hamper effectiveness.
Heroic Fire Rescue: Don't do it if you're not a firefighter. Firefighters have experience with fire conditions, protective gear, training - and even they sometimes die or get seriously injured rescuing people from fires.
Hollywood Drowning: Real drowning victims may not be able to scream for help, and could easily die even if there are people nearby - simply because these people don't look for the right signs.
Hollywood Fire: It's not the fire that usually kills, it's the smoke. Also, fire creates incredibly hot temperatures within seconds (and not just within the flame itself), and within one to two minutes something can be fully engulfed, unlike in the movies.
Hollywood Heart Attack: The Hollywood portrayal of a heart attack has caused many people to mistake or outright ignore the symptoms for them in real life.
Hysterical Woman: This Discredited Trope, now reborn in a more "respectable" sense as claiming that women are "emotional" and have more psychosomatic ailments, leads to death and prolonged suffering in womennote or, sometimes, flamboyant or androgynous gay or bisexual men who actually do have physical ailments that are dismissed as "all in your head."
If It Tastes Bad, It Must Be Good for You: This trope has led to the idea that healthy food must taste bland or bad, and as a result, people eat more "flavorful" and "tasty" junk food rather than create or find recipes for delicious/flavorful/spicy healthy food.
Improvised Zipline: Real ziplines take a while to construct and require specific, strong materials so they don't break. The cords are designed to support the weight of an elephant or two for a reason.
Impossible Hourglass Figure: The pursuit of this by many women has led to everything from dangerous diets and the development of eating disorders to unnecessary plastic surgery, not to mention lots of wasted money.
Incredibly Obvious Bomb: Terrorist bombs (as opposed to say, dynamite used legally in mining and construction) and military land mines are usually engineered to not be obvious, because their effectiveness and fatality relies upon their not being detected until they explode. Nor do they generally tick. Odd smells, strange wires, and objects that are out of place or suspicious are far more likely to be a bomb. In the case of mines/minefields, if they are unmarked (and there are a variety of markings - it is a very good idea to look these up before going to a possibly mined area), watch out for dead animals, damaged vehicles, avoided areas, and wires or suspicious ground formations.
Instant Emergency Response: In most moderate-sized cities, fire and police response is around four to five minutes with EMS being around six. In major urban areas add a few more minutes. In sparse rural areas and/or during a busy night or disaster, times of 20 minutes to an hour even for immediate situations are not unknown. The only time instant emergency response happens is due to luck. This is why learning at least basic first aid is a very good idea no matter where you are, and why it's always a good idea to call for help sooner than later (e.g. it's better to have the police on their way while the burglar is trying the door handles than when he's smashed the window and is inside, and better to have the fire department on the way when you first smell smoke/burning rather than when the room is about to be fully engulfed in flames). It's also one of the more contentious issues in the gun control debate.
Instant Sedation: A drug that might instantaneously make someone unconscious is also most likely to kill them because of its potency that slows or stops nervous system or vital functions far too quickly or effectively. On the flip side, don't expect something tranquilized to go down immediately - it takes time for it to spread through the bloodstream and leaves the target conscious long enough to be very, very annoyed with you.
Jammed Seatbelts: Seat-belts and shoulder belts do jam occasionally (or worse, melt), but nowhere near as often as they do on TV or in the movies, and fear of this causes people to not wear them and instead go headfirst through a windshield. If you must worry about it, keep a knife or seatbelt cutting tool in the glove compartment. The only exceptions to this are ice-road travel or travel over a bridge of questionable integrity over a body of water where the risk of a breakthrough into the water is higher than that of a vehicle collision - in which case the concern isn't even jammed seatbelts, but that wearing a seatbelt will add time to your escape from frigid or deep water.
Jar Potty: Unless you have no other choice, do not use a chamber pot/jar/similar for fecal matter, especially if you have diarrhea. This is how cholera became an epidemic illness, one way Ebola spreads, and is very conducive to the spread of too many other contagious illnesses to list. note On the other hand, using one for urine only is relatively safe, unless the user has a urinary tract infection or schistosomiasis infection. If there are no toilets of any sort and no specifically designed containment bags, the next best option is to dig or have someone dig a latrine (preferably far from water sources). If you absolutely must use a jar or chamber pot for feces: note never share it (each person should have their own), keep it tightly sealed between uses, never dump it in/on anything but a toilet or a properly dug latrine pit, and disinfect with bleach, alcohol, or fire after dumping if at all possible.
Law of Inverse Recoil: If you don't account for recoil, you WILL lose control of your gun and shoot off-target — maybe even someone. The heavier caliber guns can also hit you in the face. And, as the MythBusters proved, you should NOT adjust your grip by putting your thumb next to the cylinder of the revolver. That will break or cut off your thumb and give you severe burns.
Laxative Prank: This is, in fact, a criminal offense in most jurisdictions, and a particularly dangerous one as the victim can become dangerously dehydrated, have a dangerous drug interaction, or even experience a Ruptured Appendix or a bowel rupture. This goes for other substances (such as eyedrops) that are known for causing a laxative effect.
Little Useless Gun: Any gun can injure or kill, regardless of size and/or caliber. There have been cases of people being shot with small-caliber pistols, brushing it off because it didn't inflict as much pain as a larger bullet, and dying from undetected internal damage.
Lodged Blade Recycling: If you are ever stabbed, do not pull out the blade yourself as it is possibly the only thing keeping you from bleeding out and dying on the spot.
Magic Plastic Surgery: Real plastic surgery causes scarring and requires extensive and painful recovery time, things they don't show you on TV. Repeated operations can ruin the skin and facial structure. Also, it's surgery, and invasive surgery at that.
A Man Is Always Eager: The wide-spread and culturally ingrained idea that a man cannot be raped because he unable to "not want" or "refuse" sex is one of the reasons why the millions of male rape victims around the world cannot get the assistance they so desperately need after such a life-changing and traumatic event because they simply are not believed when they try to come forward and report their attack. And this disbelief doubles or triples if their attacker is a woman.
Missing Backblast: Firing a rocket launcher or a recoil-less rifle in an enclosed space will kill the person firing it and those around him or her.
Mismeasurement: An all too common cause of death and serious injury in Real Life. It's always a good idea, especially when dealing with chemicals (especially consumed drugs or medications, or anything that can cause a fire or explosion) to make sure the measurements are correct, and to always assume less is better. Also, when working with extremely strong acids or bases, add them to the water, not the other way around, regardless of measurements.
Mooning: The resulting charges can range from public indecency to second-degree sexual assault, especially if children are nearby, and there's nothing like being on a registered sex offender list.
Motorcycle Jousting: Unless you are a trained stunt rider on a closed course, this is a very good way to unbalance or totally lose control of your bike and crash.
No Medication for Me: Going off your meds unless recommended by a certified doctor isn't advised, especially if you're mentally ill, or have certain physical illnesses (such as insulin-dependent diabetes or HIV/AIDS) where there is no cure as of this date and the medication is keeping you alive, even if it's a burden to use properly or its side effects suck. In addition, withdrawal symptoms for some medications can include things such as seizures, so you should NEVER just suddenly stop taking medication.
On top of that, there's antibiotics. Going off of antibiotics too soon, or only taking them when you feel bad as opposed to as often as you're told to, can result in the germ becoming resistant to antibiotics, and then those strains spread. Antibiotic-resistant strains of many diseases already exist - and some of them are Nightmare Fuel and only aren't killing people like they did in centuries past because these drugs are reliable. Usually. For a little longer.
Benzodiazipine drugs (Valium, Xanax, Klonopin) when used longer than a certain period of time, and alcohol in longterm dependent alcoholics/heavy drinkers must never be quit immediately. All carry potentially fatal withdrawal syndromes.
Non-Fatal Explosions: Real Life explosions kill and injure. If people aren't killed or injured in one, it is solely due to luck. If nothing else, your ears can suffer permanent damage with burst eardrums being one of the most common explosion effects. The blast wave of an explosion does most of the damage and it is why someone who looks "fine" can actually have fatal internal injuries.
Nose Shove: Pushing things up your nose or into your ears is not advisable because of risk of injury.
One Drink Will Kill the Baby: While pregnant women are advised to eliminate alcohol in the first three months of their pregnancy, much media exaggerates the danger. As a result, there are a number of cases of women who accidentally took a drink and then remembered they were pregnant going to dangerous lengths to attempt to induce vomiting to "save the child."
One-Man Army: There's a reason why most military organizations emphasize teamwork, and why police call for backup. Many Real Life one man armies had help and almost always got wounded or killed.
Only a Flesh Wound: Injuries that don't involve vital organs and are left untreated can still result in death from blood loss, infection, or permanent debilitation, and they leave permanent scarring.
Only Bad Guys Call Their Lawyers: Always call a lawyer if you're accused of anything with consequences above a citation. If you are ever placed under arrest, you need a lawyer, and sometimes one is a good idea even if you're not arrested but are being questioned or even if you're only a witness. As has been noted in many places online, talking to the police without a lawyer unless you are a victim (and sometimes even then) is a very bad idea. On a related note, if your country allows silence as a right, invoke and use that right until/unless your lawyer advises you otherwise. If you are involved in a lengthy interrogation without a lawyer, demand one and don't answer questions if possible.
Outrun the Fireball: Explosions, overpressure, and debris move faster than you can run, and the shockwave travels faster and farther than visible effects of the explosion. There may not even be a fireball.
Over-the-Shoulder Carry: Unless there is absolutely no alternative, do not carry someone like this if the accident/situation could have possibly injured their neck or spine, or if they have preexisting neck/spinal injuries. It will cause more damage to their neck and/or back. See Bridal Carry above for another way not to ever carry people with neck or spine injury.
Pædo Hunt: Can lead to adults (usually men) not assisting lost or distressed children for fear of being labeled a paedophile, with tragic consequences. Also a major issue when it comes to finding men to work with or volunteer with children, due to presumptions about their reasons for volunteering and/or fear of a legitimate complaint being taken to police and a need to defend themselves, which kills the institution's reputation even if the person is found innocent or the guilty perpetrator was acting solely on their own. This is a particular problem for mentor programs, as many of the boys in these programs have a desperate desire for a proper male role-model, due to a lack of a father figure, but there are very few men willing to mentor, and some of the organizations are hesitant to take the risk with those who do.
Pants Positive Safety: Carrying a loaded firearm in anything but a holster specified for that firearm can get you or others shot.
Police Are Useless: Unless you're certain every cop in your city is corrupt, if a situation already involves violence or threats of it or the loss of property that could be used in a later crime (such as a firearm or even a computer or phone or identification documents) call the cops.
Pull the I.V.: Done wrong, it's an invitation to bacterial infection, septicemia, and traumatic bleeding. Let the nurse/doctor/trained person do it.
Put Down Your Gun and Step Away: If you're asked to do this (by anyone other than a law enforcement officer, obviously), he probably just wants to shoot you.
Don't do it. The mass of even a small trolley (much less a fully loaded cross-country train) and the speed at which it travels means that a train takes far longer to brake even with full braking power applied (which can well derail the train itself, note which is why trains in less than a few miles often don't brake for a person or even a vehicle that suddenly appears on the tracks, because the engineer would rather one person die - that likely would anyway, since a full stop wouldn't be fast enough to save them - than however many would in a crash/derailment ). Do not linger on actively used train tracks for any reason, and if a signal is warning of an incoming train (or if there is no signal but you actually see the lights or hear the noise of one distantly) do not get on or cross the tracks until it has passed.
If you are in the unfortunate situation of your vehicle having stalled on train tracks, get out of it if it won't start, especially if the signal is going or you hear a train, and get off the tracks and call 911 to report your location and that there is a stalled vehicle on the tracks. Better to lose your car than your life.
Ramp Jump: Trying this stunt with an ordinary car will almost always cause a crash with the same effect as a head-on collision because the nose of the car hits the ground first, and if it doesn't the car's still likely to be trashed. Motorcycles tend to skid or flip on the landing.
Rearing Horse: This is the most dangerous vice known to equestrianism, and anyone in doubt as to whether they're qualified to ride a horse that does this is most certainly not so qualified. A rearing horse is a horse very likely to go over backwards, breaking its bones and its rider's.
Safety Gear Is Cowardly: There's a very good reason to wear safety gear, even if it's "useless". The people that don't will get injured or killed because they were not wearing something that could've prevented their death.
Shot to the Heart: If your friend is going into shock, DO NOT, under any circumstances, stab him in the heart with a giant needle.
Soft Glass: Normal glass can be far harder than you'd think, and breaks with nasty sharp edges besides. More often, it does not break and will injure you. That's why any film or television studio worth their salt will use the classic "sugar glass" so that actors doing stunt work don't get seriously injured.
Soft Water: A fall into water may cause less injury than hitting solid concrete (thanks again, MythBusters), but it will still be lethal from a significant height, resulting in similar crush injuries. Also, if you are so injured you can't swim from the impact, you will drown, where if a fall onto even solid concrete left you alive, you might have a chance of survival with being found and getting prompt medical treatment.
STD Immunity: You don't have it. Always use a fresh new condom if you aren't with a monogamous partner. Disease transmission can also occur through open sores and other skin lesions, plus saliva (but rarely AIDS). Immunity is very rare and practically impossible to be immune to more than one.
Steel Ear Drums: Sufficiently loud noises will damage your hearing, possibly permanently. There is a reason musicians including singers tend to wear ear protection when they rehearse or even sometimes play live - and why hard rock / heavy metal singers, who often can't wear effective ear protection live, often have the highest rates of hearing impairment (up to and including ruptured eardrums) as a career-related injury.
Stray Shots Strike Nothing: Just because a bullet doesn't hit its target doesn't mean it goes away — even ignoring property damage, hundreds are injured or killed every year by gunfire that wasn't aimed at them.
Suck Out the Poison: Trying to suck the venom out of a snakebite wound is highly ineffective in almost every case, and will often increase the victim's risk of infection and the first-aiders' risk of poisoning.
Super Window Jump: Again, no matter how fast you jump through it, it can still cut you. If you punch a window and it breaks, you are lucky if you aren't bleeding... or if it shatters. If you absolutely have to break a window to escape danger, throwing a heavy object at it or kicking it with the steel toe of a protective boot is a better option.
Tap on the Head: A blow to the head is more likely to kill or lead to long term injury than temporary unconsciousness. At best, you'll get a concussion.
Trash Landing: Glass and sharp objects are commonly found in garbage bins. Safer than concrete or glass, but only should be attempted, as the page says, if the alternative is hard concrete. On the other hand, the manure bin/pile variant, while disgusting, is actually a good idea - loose dry manure IS a soft surface (loose liquid manure, however, has all the attributes of fictional quicksand).
Trojan Gauntlet: The trope, while humorous, can leave viewers with the idea that getting condoms in Real Life is a difficult, embarrassing experience - which couldn't be further from the truth if it tried to be, anytime after The Eighties or so in most of the modern world. At the time of this writing, getting condoms anywhere but in the most religiously conservative societies is very simple, and there are even places that have them for free.
Variable Terminal Velocity: All things fall at the same speed, regardless of mass, weight, or other factors save for air resistance. Don't jump after someone who's fallen off a ledge hoping to save their life, not even if you've got a parachute. You won't catch up. (The only case where it might possibly work is a high-altitude airplane jump—emphasis on high-altitude—but that presents its own problems and is definitely not safe.)
Weapon for Intimidation: Sometimes this does work in Real Life, but other times it makes things much worse. One should only threaten someone if your life is at risk and you are willing to face the consequences of following through on your threat.
William Telling: Shooting a bullet or arrow into an object on someone's head risks missing and killing them with a headshot. William S. Burroughs actually killed his wife doing the trick with a drinking glass (even more hazardous because the glass would likely shatter...)
Worst Aid: If all you know about first aid is what you've seen on TV, you are not the best person to administer it.