Conversion disorder. Basically, huge amounts of stress make your brain incapacitate you in some way to keep you from continuing to do the things that are putting so much stress on you.

Pathological Demand Avoidance.

Very roughly, it's an inability to do certain (often very basic) tasks to the point that the individual in question may shut down and withdraw entirely. It's beyond simply being lazy or 'not wanting to,' and to a point where it's a very real damage to the person's quality of life and goals.

Aphantasia (I have it!) I can't see mental images whatsoever. The condition is a spectrum, and it's estimated that 3-5% of the population has it. My memories are based on every sense other than sight, as my brain doesn't "recall" memories as a normal person does.

Munchausen syndrome is when someone tries to get attention and sympathy by falsifying, inducing, and/or exaggerating an illness.

Emphasis on inducing. Some people do things to make themselves sick.

Smoking stool syndrome.

If you eat enough white phosphorous, your poop emits smoke and can even glow in the dark. You also die horribly, so don't try it for funsies.

I have Dissociative identity disorder.

People think it's a joke or an act.

Hyperthymesia, very powerful first-person memory.

Korsakoff's Syndrome

A severe vitamin B deficiency impedes a person's ability to form memories. In order to recall things, the person's brain draws from nearby clues to fabricate a story. The patient has no idea that the story is completely fabricated.

It's treatable and often seen in alcoholic patients and patients with severe eating disorders

Defecation syncope: The temporary loss of consciousness upon defecating. Or in laymen’s terms, passing out from pooping

I have Unspecified hyper insomnia. I go up to 11 days without sleep. Sleep a couple of hours and then it starts again. I have had this issue for over 10 years at this point. I have had multiple sleep studies.

My brain waves that tell your brain to be awake and sleep are constantly running. Meaning I am literally asleep and awake at the same time.

Exploding Head Syndrome. I suffer from this hard. Basically I will hear very real auditory explosions randomly more often at night. Living alone on the rough side of town I’ve freaked myself out. My dog is my lifeline: if she is calm and peaceful, I know what I heard was an episode in my head

Ehlers Danlos Syndrome. Genetic connective tissue disorder that has a very high pass rate per child and can cause sudden death or daily joint dislocations and more depending on the type.

Ocular albinism, the part of your eye that's supposed to block light has no pigment in it, meaning that you can't really see in bright light but can see well in the dark like an owl or cat might

I saw an interview with someone who’s allergic to water

ACHOO Syndrome

It's when you have the genetic trait to sneeze at a sudden or intense light. I have this condition.

Cotard's Syndrome. Among other symptoms, you believe that you are dead. You may believe that people around you are dead. You believe these things to the point where you might stop eating (because you're "already dead") or try to go to the morgue to be embalmed and buried.

Fibrodysplasia Ossificans Progressiva, affectionately known as "FOP". It's an extremely rare genetic disease, almost 100% fatal, in which the sufferers soft tissue slowly turns into bone.

Situs Inversus. Some or all of your internal organs are reversed in position (left to right).

Basically, a person could have their heart be slightly on their right side instead of the left.

Auto-brewery syndrome which is how it sounds, alcohol is produced in the gut after eating/drinking carbs.

Restless Leg Syndrome

Persistent Genital Arousal sounds like something someone would say as a joke, but having seen a couple of documentary series cover this, it seems horrible.

Familial Fatal Insomnia, where one day you lose the ability to sleep and then you stay awake for the rest of your life until you die of sleep deprivation

Phantom hand syndrome, where amputees can still 'feel' missing limbs

Prosopagnosia, where you can't recognize faces anymore, including your own

Capgras Syndrome... people think that people they know have been replaced by imposters looking like the people they know.