Last year I observed a little leafhopper and the leafhopper IDers had to add the species to iNat.
What’s kind of funny is that according to one book that I read after it was IDed, the species is fairly widespread. Probably just escapes notice because of how tiny it is.
I seem to have gotten the first iNat observation of Phaeochoropsis neowashingtoniae - I had to add the species!
https://www.inaturalist.org/observations/107466010
I found it while I was waiting in the parking lot for my cat to finish getting her vet checkup. Not the place you’d expect to find an unobserved species.
I maintain a list of species & subspecies for which I have the only observations, currently numbering 29: https://www.inaturalist.org/lists/3795739-iNat-Onlies
This includes some observations that are not Research Grade because there’s only one identifier, so take it with a grain of salt. Also, I have multiple observations of several of these, so it’s not exactly the same as “one and only one”.
These are some amazing observations with great pictures!
Not much that I see like that. Often times I’ll see a first for Atlantic Canada or something, but never global.
My firsts for which there have been no follow-ups are
Gryllotalpa major Prairie Mole-Cricket (technically at two obs, but it was the same individual recorded by myself and another iNatter)
Stenotrema pilsbryi Rich Mountain Slitmouth
Funnily enough, I stumbled upon the Rich Mountain Slitmouth while looking for salamanders in a place outside the described range, so I barely took a picture of it and didn’t realize what it was at the time. However, I failed to find any while deliberately looking for slitmouths at suitable sites on Rich Mountain a couple of months later
After some trial and error, another one joins the list https://www.inaturalist.org/observations/30219374
The Hospe Mullet, my first “one, and only one”
https://www.inaturalist.org/observations/69262121
I hope to find many more!
just for grins, here are the only-one-of-taxon iNat observations based in Texas, as they exist on 2022-03-29:
Mine was American Whitebelt. It stood as the only one on iNaturalist for seven months. Now there are 19 observations. The reason I got it was because of a recent taxon split. Ontario is a difficult place from which to add new species for iNaturalist.
I just got another iNat first: Peronospora claytoniae! I doubt it’s an uncommon species, but it is probably overlooked by most:
https://www.inaturalist.org/observations/109902851
This one is a bit tricky to name beyond Gelechiidae, but I’d give good odds that it is the only one here. 
I just discovered another one:
https://www.inaturalist.org/taxa/79270-Streptanthus-oliganthus
Make that two:
https://www.inaturalist.org/taxa/159816-Canbya-aurea
Thanks to Peter Oboyski at the Essig Museum, I found first records of Heliogemma preclara on iNat. Hoping to add a few more species soon, if I can confirm descriptions of these colorful sun moths.
CalPhotos (berkeley.edu)
Family Heliodinidae (Sun Moths) from Bahía de Banderas, Nay., México on September 01, 2020 at 09:41 AM by Luis Daniel Santana Moreno · iNaturalist
Potential new moth record in South America https://www.inaturalist.org/observations/104242705
I was browsing around looking for interesting observations just out of curiosity and stumbled upon another one: iNat’s first and only Malvinius compressiventris!
I’m a wee bit excited for this wildflower - Penstemon vizcainensis: https://www.inaturalist.org/observations/113091559
I had a funny situation with this worm that has 2 observations, both done by me with 12 years apart (and whole continent between them). https://www.inaturalist.org/taxa/319833-Alitta-brandti
Yet both in Russia.
You have covered a lot of ground. GBIF has them in my home area too. I’ll have to keep an eye out when I do my benthic and pelagic observations.
Right, and they’re both huge and common in areas where they live, so it’s a mystery why nobody observe them.