There was a study in Russia of a man (left unnamed, referred to as Mr. X in the study) who had hyperthymesia, meaning he was born without the ability to forget anything. Phone numbers, bank accounts, scripts and books, snippets of conversation, emotions both good and bad, sensations both good and bad... well, you get the picture. He was almost normal until his mid-20s, when the information overload started getting to him. He committed suicide at the age of 32.
Jill Price is famous for having the same ability as the man above. Initially, the ability to remember everything in your life sounds tremendously useful. Teacher gives a lecture? Hey, no problem, you remember it! Unfortunately, it doesn't work that way. Jill's ability, as with most people who have this, is autobiographical memory only. She can remember that on X date thirty years before she was sitting in a high-school class at 10:15 AM listening to a lecture on history. But she can't remember the lecture itself, she only remembers where she was, what she was doing, what she was wearing, etc. — autobiographical information. Actual useful information? No.
Cells in the body, for various reasons, can only divide a certain number of times; this is essentially why we age. But some cells are blessed with the ability to keep dividing forever! The downside? Well, they're called cancer cells.
Henrietta Lacks found a way to live forever — as a spreading strain of immortal human cells! Now the HeLa cells are seen as more of a nuisance since they're apparently all over the place — and they like to corrupt other samples of other cells.
And note that even the 'found' part was much less cheerful then it sounds. Namely, Lacks got cancer, doctors took her cells for diagnosis, but ended up using them for extensive biological testing- without her permission. She died in poverty, without ever knowing that her cell line was being used all around the world, often used to help develop very profitable drug.
People with the rare condition CIPA: Congenital Insensibility to Pain, which essentially takes the form of a complete loss of the sense of touch. Can't feel heat or cold, aren't even slowed down by sucking chest wounds, pretty awesome crimefighting power right? Well, it would be if they also had a healing factor to go with it. As is, they risk permanently destroying their bodies at any time because they don't even notice it's hurt. Or that their hand in sizzling on the grill at the moment. Or that they haven't eaten in 5 days. Or that they need to pee. On a hot day, they may pass out or even die because their body doesn't figure it should be sweating.
There's an article on the NYT about a girl with CIPA. Imagine having a child who's chewed off part of her own tongue, or runs on broken ankles, or sits covered in biting ants. There's then an anecdote from an adult woman with CIPA whose pelvis shattered during childbirth and was bleeding internally, and only felt a kind of stiffness.
On the other hand, blood plasma inverts the recipient/donor relationship. Oľ blood plasma can only be donated to other Oľ people and Oľ people can take blood plasma of any type. AB+ blood plasma becomes the universal donor and also can only take blood plasma from other AB+ donors. It's good to donate blood whatever your blood type is!
Savant syndrome, in come cases. The notable being Kim Peek (the "real-life" basis for the Rain Man).
Having absolute pitch has a few downsides, leading some people to regard it as this. One twentieth-century musicologist actually tried to get rid of it, by playing the developments of Beethoven piano sonatas over and over in all possible keys.
Relative pitch really helps when it comes to singing, especially from memory. It sucks whenever someone is singing around a person with relative pitch, and the singer is slightly (or not-so-slightly) flat or sharp. The person with relative pitch can ALWAYS tell.
Lizzie Velasquez is a woman who suffers from a rare disease called neonatal progeroid syndrome (NPS). She consumes as much as 8,000 calories a day, yet is still extremely thin at only 56 lbs (25.4 kg or 4 stone).
Brooke Greenberg may hold the secret to immortality, or the very least how aging works. She has stayed the same physical and mental age for years. That age? 9 months to 1 year old. Unfortunately she passed away on October 23, 2013, but due to a disease unrelated to her age (actually it's semi-related, but it's because it affects all toddlers biologically 9 months to a year old).
There are genetic conditions which result in increased muscle mass in comparison to the general public. The downside? Your heart and lungs do not scale accordingly, putting constant increased stress on them, which guarantees a shorter life expectancy.
Conversely, anacondas and other large constrictors can swallow prey as big as they are, and go for months without eating. The catch? They need that downtime in order to regenerate their stomach lining, which tears itself apart with ulcers trying to digest its meal before it starts rotting inside the snake.
Flying is generally considered awesome. Unfortunately for the birds, this requires extensive adaptation with hollow, fragile bones, extensive lungs, and a hyperactive diet to fuel everything (sparrows are said to eat twentyfive times its own weight each day). They literally have no room for anything else but flying.
Synesthesia. This is when peoples' senses are linked in intricate ways, like seeing a different colour every time you hear a different sound, or associating a certain emotion with a number. The variations are possibly endless. Sounds cool at first, but it can also be negative. For example, you might taste chocolate every time you saw the colour brown, but what if you associated it with poop instead?
Even if you do associate the colour brown with chocolate, you have to remember not everyone likes chocolate so that could be a negative experience, probably not as bad as poop, but...
Louis Gossett Jr. said his Best Supporting Actor Academy Award (for An Officer and a Gentleman) ended up being this. Sure, an Oscar is a wonderful achievement for an actor, but Gossett said that it put him in acting limbo, since smaller productions didn't think they could afford him (since he's an Oscar winner) and big productions didn't think he could carry a movie on his own (since he won for Supporting Actor). A lot of Best Supporting Actor/Actress winners have trouble finding work afterwards.
Sir Alec Guinness's work in Star Wars. He was perhaps the only person associated with the film, aside from George Lucas, who actually thought the film would be successful, but he never imagined it would become one of the biggest movies of all time, overshadowing the entire rest of his career to the point where he ended up equated in the popular consciousness exclusively with the character of Obi-Wan Kenobi. He was not happy about that bit.
Being a genius also sucks. A lot of people with extreme high I.Q.'s or intelligence suffer from depression because they don't have anybody on their level to talk to. Many also don't see the point of using their talent for higher benefits as they would rather have a normal life. And a lot of people with extreme high intelligence are often loners who have trouble living together with more average people.
Gifted children are especially vulnerable to this trope, since they seem to be in a sort of lose-lose situation. Either they are encouraged to stimulate their intellectual capacities (meaning that they have to skip entire years of school and go directly to University at an early age, meaning they have to be in a completely foreign environment for them) or they are kept in their school year (which, while benefits them due to the fact that they can interact with children their age, can cause them to be problematic, since they are sitting in a class learning things they already know).
There's also the tendency of gifted kids to grow into extreme motivation problems. They don't usually have to work very hard in school because the material is easy, and are rarely rewarded proportionally for it because they can't do better than "perfect" and "perfect" is their "average". This means that if they ever do find something difficult, they feel like failures because they're supposed to be smart enough to handle anything, or get put under such pressure to succeed that they run themselves ragged just to avoid the punishment that comes from being less than perfect. This usually ends with the gifted kid growing into an adult who shuts down when something doesn't come easily, either because they feel like a failure and are too ashamed to continue, or because they've been doing perfectly all their lives and have nothing to show for it except being eligible to be expected to be perfect somewhere else.
A rare genetic condition called Urbach-Wiethe disease can, in some cases, cause damage to the amygdala, dampening or removing the victim's ability to feel fear. Which, on top of all the dermatological and neurological problems caused by Urbach-Wiethe diease, means you're not afraid of anything... including things that could be potentially dangerous.
Having a job that involves frequent travel. On the one hand being able to visit all sorts of other places and having your employer pay for it all sounds great...but this also will uproot you from home and your family quite a bit, makes scheduling your social life far more complicated and can make real vacations less enjoyable.