A lot already mentioned, so those too, but also:

Rogue black holes. Just thinking about them makes my guts queasy.

If enough psychologists and civil servants wanted to, they could have you declared insane, and then nothing you say can come to your defense as you are now deemed too insane to be listened to. I know this isnt as straight forward as the average person thinks, but the possibility of it even happening once to someone is terrifying.

Oh that and trigeminal nueralgia, the worst pain possible forever.

transmissible cancer, some surgeons had got cancers from thier patients by accidentally pricking themselves with a contaminiated instrument, although it was treatable and non issue once the tumor was removed. other forms like transplant patients is another source. rare t-cell cancers like mycosis fungoides, you go from patches, to "plaques" to tumors.

and there has only one case of animal to human cancer, the guy had AIDS and got tapeworm cancer from it, which is the most unusual situations out there.

Snake clowns!

Sometimes I think of the literal worst thing that could happen.

Like, going to hell, and being told you can leave if you complete some fucked up, borderline impossible task, like:

• Make friends with everyone else who is in hell. Not get them to like you. You must like them too, and enjoy their company. Yes, even Mother Teresa.
• Consecutively live through everyone's life who has ever lived, as a passenger in their brain, you can't control anything, but you experience all physical and emotional pain.
• Suck all sand until the earth is sandless. You have to suck each grain one at a time. Also, you have to find them, they're not just in one big pile, also, there's sand at the bottom of the Marianas trench
• Live your life on repeat until you live it perfectly. What does perfectly mean? You're not told. Also, you're not given any feedback after you die each time. If you try to cheat the system by having fun, you'll be yanked back to hell and raped by demons before starting back at day 1.
• Achieve universal human genocide in 100 different times periods and starting points. Also every person you kill also ends up in hell, and know its your fault.
• Learn every vim function, then be forced to rewrite all the world's code in nano.
• Translate every book, movie, song, and poem ever written into every other language, even dead languages.
• Think of a colour which doesn't exist, then invent a new instrument, and play a song with a tune which has never been performed, invent a new language with a completely new structure, and convince everyone else in hell to adopt it as their primary language.
• read and reply to every comment on the internet. If someone replies, then you need to reply back. Every message must be unique and coherent. Also, spacetime warps to make it possible for everyone to be given a chance to reply to you.

On the smaller scale it's probably my wife and dog dying, in the larger scheme of things it's the rise of fascism here in Germany and Europe.

Not having any feeling in your legs, but not being paraplegic, so you could legit accidentally walk outside without any pants on.

Some form of torture. The thing about torture is you can always add "and then..."

Psychological and physical

Imagine being forced to rip off your significant other's fingernail.

And you might say "There's no way I'd do that"

Sadly you will. Either to save yourself or them from more torture.

You will do it, we all would. That's scary.

Prions - Actually nightmarish.

Getting falsely accused of something that requires me to interface/deal with, and potentially be incarcerated within, the US justice system. Fuck all that with a bundle of rusty rebar.

The worst is that it can possibly happen at any given moment. You could look at a redcap Karen in a store, they take it the wrong way, and bam, your life is a loving hell for the next decade or more. Nope.com

Nice try, sleep paralysis demon

Mondays

alzheimers.

Abuse of the psych. bug that leads to faschism by smart evil people that know exactly what they are doing and somehow make it work stable for centuries.

Dying painfully

Cancer. Everyone has a bunch of fucked up cells at all times, but your body keeps you healthy by repairing or killing them, time after time after time… until one of those cells says “fuck the system” and goes rogue. No one is safe.

Imagine every day, your muscles get just a little bit weaker. Maybe you can't open a container you used to be able to. Or you find you can't lift your arms high enough to shampoo your hair anymore. No matter how much you try to exercise, it never helps. Pain sets in due to incredible stiffness. Your fingers start to curl up until they are nigh unusable. Entire limbs become gnarled.

Eventually you lose the ability to walk. Then speak. Then eat. And finally, even your breathing muscles become so weak and paralyzed that you constantly aspirate due to your inability to cough. Recurrent infections set in. Then you die of respiratory failure.

Being given an anesthetic that should put you out for an operation but only immobilizes you and you remain cognizant thru out the operation feeling everything being done.

Kinda happened to me in middle school when I broke my arm and they gave me anesthetics to knock me out so they could re-set my ulna and radius but half way thru i wake up to my arm in 5 Chinese finger trap-like device to hold my arm up and a strap across my bicep with weights on it weighing my arm down and 2 doctor's trying to push my bones back in line.

unit 731 (seriously, genuinely, don't look it up if you're unfamiliar, it's beyond nightmare fuel the kinds of torture imperial japan committed and got away with)

The idea of lobotomies.
Just being trapped in a shell of who you once were, for the remainder of your life... Bloody hell, what a horrible part of history.

That we may be living in a world where the super-rich and powerful from all countries have conspired to establish a kind of global absolutist monarchy of the utterly unscrupulous, in which fabricated conflicts between countries serve mostly to distract from the fact that those pulling the strings behind the scenes have already undermined every political system and are simply pitting people against each other in order to secure even more power for themselves through war, mass surveillance, propaganda, the rationing of essential goods, and blatant destruction.

Please note: This is a conspiracy theory with no solid evidence, not a fact, but a dystopia that unfortunately seems increasingly realistic these days.

Bears. They don’t kill you right away, they just crush your face and start gobbling at your gutty works. Sometimes they leave for a bit to digest while you’re still not dead and come back to finish their meal later.

There are existential horrors and real life scenarios that are probably worse than death, but bears still take the cake for me.

In terms of anything that does not include something related to death or torture its probably having an operation on your eye while being conscious. I know someone who had to get surgery on his eye and could see while two doctors were doing stuff on his eye. Even just hearing this scared the living shit out of me.

The sun moving the other way through the sky

Glacier collapse causing sea levels to rise. Global ocean currents failing. A meteor hitting the earth. The next pandemic Locust Super Volcanoes

Emerging evidence of abrupt changes in the Antarctic environment

...evidence is emerging for rapid, interacting and sometimes self-perpetuating changes in the Antarctic environment. A regime shift has reduced Antarctic sea-ice extent far below its natural variability of past centuries, and in some respects is more abrupt, non-linear and potentially irreversible than Arctic sea-ice loss. A marked slowdown in Antarctic Overturning Circulation is expected to intensify this century and may be faster than the anticipated Atlantic Meridional Overturning Circulation slowdown. The tipping point for unstoppable ice loss from the West Antarctic Ice Sheet could be exceeded even under best-case CO2 emission reduction pathways, potentially initiating global tipping cascades...

This is just an example from today, but overall the climate news from recent years scares me more than anything else. Insect counts are plummeting globally, wildfires are getting noticeably worse everywhere, all of the ice is receding, ocean currents are destabilizing as the temperature patterns in the air and the water shift, and crop growing seasons are changing. I get the impression that the predicted climate changes are not only happening now, but are accelerating noticeably.

Unpredictable food production is bad bad bad. If you watch the series Fall of Civilizations, one of the most common elements of collapsing societies is destablised food production (not necessarily no production, just irregular). If this happens globally, everything stops and people start dying in large numbers.

Personally, this is the only political issue that really matters. Every other thing that people argue about is just rearranging the deck chairs on the Titanic.

Dick spiders.

More seriously: body parasites of any kind.