That species that you should have seen no problems, but just never find. The one that when you mention never seeing one to other local naturalists, they just go “Seriously???”
Here is mine
4 years of being back in Canada, I should have seen this many times over, yet still it eludes me.
For me, I’d imagine it would have to be any of the following:
Periodical cicada
Golden Eagle
Red-throated Loon
American badger
I still don’t have a Pinyon Jay, even though I live in New Mexico. Also, even though I live in a desert, I have yet to see a single scorpion or tarantula species ever.
Probably some of the common tree species in this area. There are many I have not really learned to discriminate yet.
It’s amazing how hard it is to spot a badger (at least in my experience in North America). I’ve spent a ton of time outside in grasslands and shrublands from Minnesota to Idaho, to Utah to California. I can’t remember if I saw one in real life for a split second or whether I’m just remembering a moment from a nature documentary!
sometimes I suffer from “common species blindness”, ie when things are so common that I assume that they’re already on my life list when in reality they aren’t. Teddy bear cholla was one of those things for me, one of the most common and recognizable cacti in Arizona and a species I saw literally almost every day but I didn’t realize that I never photographed it until after living here for over a year.
I’m sure there are plenty of other ridiculous holes in my life list, but if I knew what they were then I’d be out trying to remedy them :)
I have some embarrassingly common ones here (Cooper’s hawk? yikes):
A nice way to find some of your gaps is to use the URL qualifier unobserved_by_user_id=, then plop in your username and refine to your place and taxa of interest. Also the “Missions” feature on the Android app.
I don’t keep life lists, but I would love to see a specimen of Euxoa churchillensis. Only a couple of records of it, and they are all widely dispersed. Lafontaine believes this was caused by glaciation. http://mothphotographersgroup.msstate.edu/species.php?hodges=10711
Belocephalus, Phanaeus spp. (rainbow dung beetles), probably more…
Most large species of Dynastinae and Lucanidae in my area have eluded me despite years of searching.
Our camp site at Joshua Tree a few years ago had to be moved due to a badger burrow. I thought for sure this was my chance bit even street checking it multiple times, no badger. Boo.
I just tried Cassi’s url with my location and ID, and I got a surprise… I haven’t observed Dacrydium cupressinum yet, even though my house is made out of timber from that tree!
It’s not exactly common in my area, but I really want to see a Mountain Beaver (Aplodontia rufa, which should actually be named the Lowland Not-a-beaver-at-all). I’ve staked out their habitat a number of times, but no luck.
There have been Nyctereutes procyonoides (Raccoon Dog) sightings at one of the royal palaces in Seoul as well as along the waterway not far from my apartment but I have yet to encounter one myself.
Is it strange that several of the spider species on my Favorites list are missing from my Life List? I’m in love with so many creatures I’ve never seen in real life. 
Carolina Chickadee (Poecile carolinensis). About a day after starting iNat, had a swarm of them on my patio (birdfeeders). In swoops a hawk, perches on a planter on the patio. It flies off before I can get my camera, but after scaring them all off.
Now they are few and far between, and won’t stand still for me to get a picture.
I have gotten a recording though. :)
Monarch butterfly is still missing from my iNat list. I know I’ve seen a few but not when I’ve had the camera.
Eastern Tiger Swallowtail would be mine. I see them all the time but they never seem to land.
