Genus Arion, I believe.
Oh goodness, I have a lot of these. A lot of them are either leaf mines hiding in plant photos, or other insects hiding in the back of multi-insect photos (such as at a UV light that tends to attract a large number of species at once).
Here’s one example that I’m uploading soon!
The tiny black and white moth at the top is not a species I saw at the time. Fortunately the subject moth was flying around a lot so I have some pictures that are not in focus…but on the other hand, it meant one of the shots showed the bystander moth in focus instead!
I’ve been trying to get better at acknowledging every individual plant in a “mat”, but sometimes I still have cases like this where I missed something interesting in full-bloom!
Recently this spurge mat in AZ actually consisted of 2 species on closer photo examination:
A smooth and hairless species:
And a hairy, serrated leaf species which I had to crop out of the background, not in focus:
For most people it might look subtle but I still always kick myself in cases like this! Cryptic diversity often leads to some of the most important observations.
Great picture! The Andrenidae could be Calliopsis subalpina.
https://www.inaturalist.org/observations/328843
When I took this picture, I saw that there was a mating pair of assassin bugs, one of which was slurping up some other kind of hemipteran. I didn’t notice at the time the pair of round beetles (I think) who were also helping themselves to the assassin bug’s meal. Nor did I notice the (at least 5) little flies crawling around on everything else.
They were stinging us in the car, not really painful though. Drawn in by the lights at night. Must be something nocturnal…
Twice now, the same species of Ambush Bug ended up in photos I was taking of other organisms. I didn’t see them while photographing, although in the first instance how could I have missed it?
https://www.inaturalist.org/observations/98035298
https://www.inaturalist.org/observations/98134665 (last photo)
Turns out I have one! I have this lovely collection of shells of the Panamic Comb Venus, Hysteroconcha lupanaria. But what I didn’t know is that one of them was an “impostor.” Can you tell which one it is?
It was pointed out to me that the shell at center right has more spikes, set closer together, than the rest. Look closely:
I am told that it is probably Hysteroconcha multispinosa. So now I have two from the same genus, from the same location. And it goes to show that diversity can hide in plain sight, if you don’t know what details to notice.
Cool project.
To get around that I’ve started making the first photo in the plant obs a heavily cropped one showing only the plant. With that plus writing it in the description plus giving it an initial ID, I usually manage to get the point across.
I often put in a note saying something like “There are multiple species in this photo. This observation is for X. If you can identify Y, please go here:” (add a link to the other obs). Adding a cropped photo as the first one seems to work best though. I will sometimes still have people commenting that “the second photo shows multiple species” and then have to explain that yes, I’m aware, and I’m trying to get IDs for all of them.
Here’s another example I found as I was processing pictures:
We were photographing lichens and I didn’t see the springtail (?) until I looked at the images on the computer so I missed my chance to get a better close-up of it. These tiny guys seem to frequently hide out in my lichen and moss pictures and only in about 1 of 4 or 5 cases do I notice them while still out in the field.
I’ve got a obs of a frog that has a ant on it, but i do not have photos if the ant. 
Fact: There are none obs of Entognatha in my country!
Sounds like something to fix! 
In keeping with this thread, there’s probably a few hiding in existing observations somewhere already. Just checked Vietnamese lichen observations though and couldn’t see any …
Just found this caterpillar in my photo of a bee from 2018
caterpillar observation: https://www.inaturalist.org/observations/108453522
Hey, did you spot the little duck next to the swan and those American coots swimming next to the tree?
This picture is awesome @Jophophora