The Jail Bait Wait: Depending on the state, you may not even have to wait. Some go as low as 14. Some have "Romeo and Juliet" clauses allowing a young adult to go ahead with an older teen. But yes, people have been known to do this.
Jaywalking Will Ruin Your Life: Some relatively small offenses that hurt no one can still get felony charges with all of their impacts on one's life, or can get one beat or shot by police. In some cases, people trying to get away with a minor offense, get caught in a Snow Ball Lie and end up jailed on perjury or other similar charges.
Jerkass: Sadly, these types of people are insanely common.
Jerkass Façade: There are some people who act mean despite the fact they're not, most commonly if they are avoiding to get themselves hurt. Then there are other reasons where acting mean happens to be a part of a job. Drill sergeants, for example, act this way due to their occupation demanding them to toughen up their recruits, and this usually by degrading them as much as possible without mercy so they will set their goal higher and become stronger because of it.
Jews Love to Argue: The Talmud itself is a result of legal arguments regarding Jewish religious laws. This emphasis may have also been a contributing factor in the amount of Jewish lawyers, stereotypical and otherwise.
Joggers Find Death: Became a trope (and an overused one) because it is truth: hikers and joggers (and walkers in general) tend to find bodies because they have reason to be out in areas where they may be dumped or where crimes may have happened and/or they aren't driving by fast in a car.
Joke Item: Plenty of them exist, some for the point of using them in practical jokes, others for many other reasons.
Joker Jury: War crimes trials held by the victorious nation can be seen as this. As can very biased juries, for racial or other reasons. Attempting to prevent this is the reason that the voir dire process exists in some countries that use juries.
Joke Weapon: Happens in Real Life. In one variant, carried to look like an actual Weapon for Intimidation but with no intent of actually being used. In another variant, used - there was at least one case of someone being slapped with a fish.
Kaleidoscope Eyes: Some people have eyes that can change with the lighting of a room - these are generally people whose eyes somehow ended up a greenish blue or bluish green, so depending on lighting they can have blue or green or blue-green. Also happens with dark-brown to black - dark browns will show up as black in some lighting but not vice-versa. Most people, however, achieve this effect with contacts.
Kaleidoscope Hair: Some people do dye their hair a lot, especially in subcultures like punk and Visual Kei. Hisashi Imai of Buck Tick and hide of X Japan, for only two examples, were almost as famous for their many and varied hairstyles as they were for their music.
Kavorka Man: Money can make any man attractive - at least to those who want to use the money and run. Being a popular artist or musician, no matter what happens to one's looks, means some of the Groupie Brigade will always be around. In a less cynical sense, some do have some charm or appeal aside from their appearance or even ability to provide financially.
Kick Them While They Are Down: In the sense of violently attacking an entirely helpless foe with the intent to kill, this can be anything from first-degree murder in a personal situation to a war crime and violation of the laws and customs of war if done during a war. In the sense of trying to make someone feel worse or make their situation worse in hopes of driving them over the Despair Event Horizon, it's one of the more cruel forms of bullying.
Killer Rabbit: A great many animals are meaner, sharper, stronger and/or more poisonous than they look. This is why you don't play with wild animals. And why you ask before playing with pets that aren't yours.
Kill Sat: The Strategic Defense Initiative, more commonly known as Project Star Wars. Similar projects are in the works, mostly because lasers are cool.
Klatchian Coffee: Exists in real life with some coffee+energy drinks, multiple espresso shots, or coffee brewed with coffee among others, and can be quite a shock to those who are caffeine sensitive or who don't regularly consume caffeine. Then there's the far more illegal "biker coffee," which consists of coffee + a dose of amphetamine or methamphetamine.
Knight Templar: Strong beliefs can drive people to do very bad things while thinking they are doing only the best/what "God" wants/etcetera.
Knuckle Cracking: And no, it doesn't actually hurt your knuckles or cause arthritis.
Kung-Shui: Some Bar Brawl incidents have done this. One legendary Visual Kei brawl in Japan burned the bar where things went down to the ground.
Lab Coat Of Science And Medicine: Seriously, lab coats are everywhere in real medical and laboratory settings—and for good reason. Besides the trope-accurate function of visually identifying medical or scientific staff, they also provide protection for the garments worn underneath them (which may be much harder to discard when soiled) and a convenient source of pockets (which may be sadly lacking in scrubs).
Laborious Laziness: The construction of "man caves" and "mom caves." Putting effort, money, or time into cheating of any kind when the cheating actually takes more time or money with similar results to not cheating. Working at professions such as day labor and film/TV extra when one can and has no reason (from other committments to being unable to get employed) not to get a steadier job.
Lack of Empathy: Antisocial personality disorder which encompasses the psychopath (lack of empathy and common sense) and The Sociopath (lack of empathy but trainable to "behave" if behaving in a certain way benefits them). Some but not all or even most autism spectrum disorders, though in autism it tends toward "lack of ability to express empathy in a way that is understood." Some schizophreniform disorders and forms of dementia, though in those it tends toward "I don't care about anything at all."
Language Barrier: People being unable to communicate due to not sharing a language is as old as civilization.
La Résistance: The French and Dutch Resistances were notoriously violent and strong, as were the Soviet and Yugoslav partisans, and the Polish Home Army.
Krav Maga, the deadliest martial art, was made specifically for Jews getting accosted by the SS.
A symptom of an active schizophrenic or manic episode in real life, called "inappropriate effect."
A more benign version occurs when someone confronted with bad news or tragedy either absolutely disbelieves it ("you've got to be kidding, don't joke about that -laughs - Oh Crap... you aren't?") or finds something darkly humorous about it and can't help but to laugh ("Wait a minute -laughs, tries to stop laughing - you are seriously telling me that while running from a pack of wild hyenas escaped from the zoo, he got run over by a bus full of rodeoclowns and dragged himself to the safety of a Port-O-John only to explode it when he lit a cigarette inside?") These people aren't schizophrenic, manic, or even mentally ill - the first, absolute disbelief, is actually one of many normal ways to respond, and the second is not necessarily indicative of mental state as much as of the incident itself being tragicomedically stupid. The way to tell the difference is that people with inappropriate effect generally don't know or care they shouldn't laugh, while someone laughing out of disbelief will usually stop when the truth of what happened sinks in, or someone laughing at a Darwin Award-worthy fate will acknowledge that they probably shouldn't be laughing but they are.
Law of Inverse Fertility: The first part of it is very common in that the more people try to concieve and worry about it, the more likely they are to destroy the relationship itself, end up sexually dysfunctional, etc.
Lean and Mean: Anorexia nervosa and starvation in general can lead to mood swings and cruelty toward others, either for their being "fat" or "lazy" or somehow else not measuring up to the starved standard, or out of the need to feel better about oneself and one's own place for which one is "working so hard."
Leeroy Jenkins: They have existed in real life firefighting and military and police operations. In fact, long before the trope namer, there was the infamous "Charge of the Light Brigade" as a military example.
Leave Behind a Pistol: Intentionally leaving a suicide method in reach of someone likely suicidal has happened.
Lensman Arms Race: World War II and the Cold War. Consider: the Germans only made very primitive payload rockets with long range work somewhat in WWII, and America couldn't launch a rocket as late as 1957. We were on the moon just over a decade later. (The Space Race being guided on both sides by genius German rocket scientists from Nazi Germany, who were unaware of each others' existence as competitors for quite some time). And nothing says "my dick is bigger kung-fu is better than yours" like the ever-more-powerful nuclear weapons tests of the 50s and 60s.
Les Collaborateurs: Has happened in many real life police states and totalitarian regimes. The Stasi was almost even worse than the Nazis with this - people collaborated with them too willingly.
Less Disturbing in Context: Why "keyword spying" by the govermnent and/or police is a bad idea, and "zero tolerance" almost as bad. There are plenty of words and actions and events in life that context makes entirely innocent. For example, "a girl in a class hands her classmate a pill" can be drug dealing or poisoning an allergic person, or it could be a person who knows her friend isn't allergic to aspirin giving her an aspirin for her headache. Someone who "threw a newspaper rack through a window" could be an angry rioter or a burglar - or someone who realized the loud sound of the alarm activating would chase off the muggers pursuing him.
Life Isn't Fair: You can expect people to say this, usually to rationalize their insensitive and unsympathetic behavior.
Life or Limb Decision: People have had to make these, from meningitis patients to trapped hikers and climbers.
Light is Good: Most human beings are diurnal and need natural daylight to properly function. Fewer crimes happen during the day (aside from house burglaries and shoplifting in open stores), and natural sunlight allows for the safest travel in most places that aren't deserts. Sunlight also allows plants to photosynthesize nutrients and humans to synthesize vitamin D.
Light Is Not Good: The flash of a nuclear bomb or of an asteroid exploding in the atmosphere can blind people, and can itself produce such intense heat as to cremate people alive and set everything it touches that is nonreflective on fire. A gamma ray burst can take out entire planets, and gamma radiation from such bursts, nukes, or other sources is carcinogenic, harmful, and can be immediately fatal in human exposure. For individuals with porphyria or with other sun-sensitive conditions, too much sun exposure can lead to disabling, painful sickness. Certain patterns of flashing light trigger epileptic seizures in those with photosensitive epilepsy, and overexposure to certain types of flourescent light can cause myriad health issues. Nocturnal people tend to work, live, and feel better sleeping during the early morning/day and awake at afternoon or night. Finally, as noted, most home burglaries and much shoplifting do happen during the daylight hours.
Like You Were Dying: Some terminal (and even potentially curable/treatable but likely terminal) patients do this, especially if they are early in the course of their disease and both physically and financially capable of doing so.
Limited Wardrobe: Happens when poverty and a specific style/interest meet. There's a reason some musicians, for example, seem to wear the same tour shirts over and over again... or why a Visual Kei band will use the same costumes or recycle costumes.
Literal-Minded: Asperger's syndrome but not always. More severe autism, more likely. Also machine translations, and those unfamiliar with a language's idiom and syntax.
Living Weapon: People have committed crimes using attack dogs and snakes.
Loan Shark: Why do people use their services instead of a, you know, bank? Bad credit, my dear Watson. Bad credit.
Locked Out of the Loop: Often happened re sexual abuse of children in older times or even bad injuries/illness in early childhood, out of fear that the child would be traumatized more (and the adult would break down completely) if told that he/she had been raped by his father or uncle or had almost died from something before his/her third birthday.
Locking MacGyver in the Store Cupboard: Surprising but true. The most memorable case would be John and Clarence Anglen's and Frank Morris' escape from Alcatraz (as hard as it is to believe someone would actually fall for the fake-dummy-head-in-the-bed trick).
Lonely at the Top: Because a lot of the behaviors needed to get and stay at the top of anything range from solitary and not aggressively prosocial (e.g. being a hardcore workaholic) to actively antisocial (e.g. fighting off attacks from competition, either violently or nonviolently). It also doesn't help that there are a lot of people who do want to undermine others or take from them - so being at the top of anything from a business to a scene to whatever also turns every possible friendship or relationship into something with trust issues.
Lonely Rich Kid: This one comes from the aforementioned trust issues that being rich (or famous, or both) provides, plus that being rich means living in a world often split from that of the rest of humanity - and being either envied or worshipped for it.
Loners Are Freaks: People often wrongly jump to this conclusion when dealing with loners in real life. Some may well be, but social ostracism doesn't make them less so in most cases.
Loner-Turned-Friend: Definitely happens - especially because many "loners" aren't antisocial or misanthropic in nature.
Long Distance Relationship: There are quite a few of these in reality. Extended military deployments or other committments, someone being unable to leave somewhere they've established themselves to start over in their lover's country/city/etcetera, or relationships that begin online (or before that, via mail) among many other reasons, can lead to this.
Loony Friends Improve Your Personality: Sometimes having a friend that isn't entirely straitlaced and ordered and centered helps one to become more social, more accepting, or see things one otherwise wouldn't. Other times, it doesn't and just damages your reputation for associating with them.
Losing the Team Spirit: Sometimes people or even things are both irreplaceable and vital to the success of a given thing that their loss sends everyone into turmoil on trying to think of how to go on without them. If, for example, a central member of a band that was with it from the beginning or near-beginning and one of its most iconic members with a unique style dies, the odds are fairly split as to whether the band will break up or go on.
Lost in Character: Method actors, Visual Kei artists, and performance artists do this on purpose. Some people who become mentally ill while acting or roleplaying can also do this, though because they've lost touch with reality and Cannot Tell Fiction from Reality.
Loud of War: Loud music or noise that one or both parties find annoying or painful has been used as everything from Cold-Blooded Torture by the US military and other militaries (a use that has drawn much protest from some of the Heavy Metal bands used for the purpose) to a tool in petty neighbor disputes.
Lost Wedding Ring: The lengths people have gone to to get them again can be quite amazing.
Lovable Alpha Bitch: Because not even all people who fall into the stereotype are hateful and angry toward everyone, and even most bullying alphas are closer to "jerk" or "narcissist" (which means they do have feelings toward others to some extent) than outright sociopaths or psychopaths.
Lovable Rogue: Because some "rogue" behaviors are actually more prosocial (at least to their immediate group) than they may seem to society at large (e.g. shoplifting groceries from a big box store might be antisocial on one level, but if all of one's family and friends are broke and one is giving the stolen goods to support them, the theft is prosocial on that level), because some people do find "rogue" or "outlaw" behavior attractive, and because again, these generally aren't psychopaths or sociopaths even if they are criminals or outlaws.
Loves My Alter Ego: Happens with fangirls and fanboys falling in love with artists/musicians, and/or other artists who don't meet them privately and know them personally, at least to begin with. The subversion happens sometimes: these people learn who the person really is, and love them the same or even more than before they knew.
Lower-Class Lout: A stereotype and a Vocal Minority at best, but unfortunately there's enough of them in the lower classes everywhere to get all lower-class people suspected of being them in some instances.
Loving a Shadow: "Loveshys" and "Involuntary Celibates (incels)". This is the problem with loving someone from afar.