[Pseudoscience-is-malarkey (OP)]

Some say when virtually no water was detected in Jupiter's atmosphere; or when the "face on mars" was a mirage or when Venus turned out to be a replica of the mythical Hell, and not the paradise planet people thought it might be.

[Halc]

Far more disappointing was the discovery that Earth was not the center of the universe and that our significance in that universe was negligible, the complete opposite of what everybody presumed for such a long time.

The face on Mars is not a mirage. A mirage is something that is really somewhere else. It is a play of shadows, something that is really there, but only looks like that in just a certain light. If that's somebody's biggest disappointment, they need bigger expectations.

[Bored chemist]

The cheese shortage on the moon.

[evan_au]

That the space race stopped in 1972. They decided it was much more productive to spend the money on wars instead.

The premise of the TV series For All Mankind was that the Russians beat the Americans to the Moon by a few weeks, and the space race continued.
- If so, we would have permanent bases on the Moon by now, and maybe even have people set foot on Mars(?)

https://www.imdb.com/title/tt7772588/

[Zer0]

To look as far as We can possibly fathom...
And Realize there is Noone else..
Except Us!

[Origin]

The biggest disappointment is that haven't found any planets that could be habitable and the realization that there may not be any.  Of course we couldn't get there if we found one anyway.

[Pseudoscience-is-malarkey (OP)]

The cheese shortage on the moon.

That implies that some cheese was found. There was no cheese found on the Moon.

---
Harry Caray: What if the Moon were made of barbeque spareribs, would you eat it?
Astronomer: What?
Harry Caray: I know I would. Heck, I’d have seconds. Then polish it off with a tall cool Budweiser.

[Bored chemist]

That implies that some cheese was found.

No, it does not.

[Zer0]

p455b.jpg (35.53 kB . 424x250 - viewed 1406 times)

[alancalverd]

Possibly the realisation that NASA developed a pressurised ballpoint pen that works in zero-g, but the Russians used propelling pencils and other neolithic technology like Soyuz, that still works.

Similarly, recovery of NASA capsules involves at least one warship with a crew of a thousand specialists to rescue the seasick astronauts, whilst Russian re-entry vehicles land in a desert whence they are recovered by a couple of blokes in a truck.

[Bored chemist]

one warship with a crew of a thousand specialists

Specialists in what, exactly?
As you say,   there are on a warship; the vast majority of the people present are specialists in naval combat.

"rescue the seasick astronauts,"
I think you will find that the space programme weeds out people susceptible to motion sickness.

in a desert whence they are recovered by a couple of blokes in a truck.

Unless that truck is an ambulance there's a fair chance that the space crew will die.
The Russians sent essentially the same sorts of specialists as the Americans did.