You could walk in there and pull the fire alarm

I think you need to take the same approach as the British during WW2 with the Enigma. They could decrypt the messages and know when attacks would happen, but if they stopped every attack, the Nazis would know and change encryption device. So you need to accept that some people will die and only mitigate the disaster in small but impactful ways.

Anonymous bomb threat

Fuck that. Start mining. Don't say a fucking word to anyone. Don't go overboard on mining. Or if you do, use it/sell some of it early. The only reason bitcoin became popular is because people used it to buy stuff. If you start talking about how it's gonna be worth 80,000 in 10 years you run the risk that more people will hoard it and it won't be worth shit.

I need a writing prompt sub/community to get right on this one.

Maybe also an evolved strain of the pandemic h1n1 virus. And more evolved versions of other viruses, too. That covid will be more evolved than the original pandemic strain, also. It'll have the transmission increases and immunity avoidance while in the novel virus stage, so going back to warn about covid could instead make it happen earlier and much worse (instead of just earlier). Plus all the other pandemics on top of that, and in a time that is much less ready to pivot office work to wfh.

Incredibly probable

What if it worked. What if like dr.stranges 1/14million chances. This is the least caustic one.

Just stop the dumbass kid from climbing over the railing. No vest needed.

We were prepared for COVID until Trump threw away the plan. https://www.statnews.com/2020/05/17/the-art-of-the-pandemic-how-donald-trump-walked-the-u-s-into-the-covid-19-era/

Prevent 9/11 probably unrealistic, but you could pull the fire alarm prior to the attack and thereby save lifes

It depends on how far back you go and who you contact about he incident and the evidence to solidify the claim. 9/11 could have been stopped relatively easily with a few days notice to national security. The air force could have shot both planes out of the sky. Just call the civilian deaths a tragic casalty of terrorism and use it to help fuel the war. The twin towers could have been shut down that day.

But you've gotta materialize right in front of the commander in chief, bring a mountain of carbon datable evidence like news papers and original classified docs, and hope that you don't get brained on the spot before you make your case.

As for covid, you probably can't stop it but maybe better warn and prepare world governments so they can get their population ready through subtle conditioning like trying to get wearing mask be a fashon trend or advertising bidets heavily as the new rich yuppie thing to show as a status symbol.