Someone brought up on Discord the “Tego” from when you take Kud and Mio to the test of courage.
By the powers of Google I found a blog explaining it being an Air reference. The word that GaoGao translates to “Violate” is 手込め[Tegome]. We obviously also have Mio doing Minagi’s iconic “Po.”
There’s a fuckton of other Air references in LB!, especially references to YerianPotato.
lol, I’d never seen it as anything but the usual joke on Kud’s English, but that’s an interesting interpretation too x)
In LB on the 19/5 at the cafeteria during lunch: Saya challenges Riki to a food buying game whoever can get the most points wins and anpan is worth the least points, a reference to CLANNAD where Nagisa only buys food when the crowds clear up and as a result all she gets is anpan.
There’s some pretty sneaky references in Little Busters I’ve noticed.
Exhibit 1: Kud has a key charm of Potato from AIR.
Exhibit 2: I can’t see this as anything but a reference to Hop Saito from Tomoyo After.
Okay, I’ve got a few here that aren’t really groundbreaking, but haven’t been shared here before for one reason or another - probably because they’re more about Tactics than Key, but whatever.
This one has nothing to do with Key and is pretty obvious, but we can see the cast of MOON. in the One cafeteria.
And in One, Kanon and Air successively, we see the “LOLICON” brand :
Edit: Apparently Makoto is a fan of the LOLICON brand.
Lastly, there are stores named ONE and MOON in the shopping district of One. If you look closely, you can also see a store called “RORIKON”.
Oh, and I almost forgot the Tactics TV in Kouhei’s living room:
Edit: Holy crap, all the other ones I’ve shown here were kind of lame but this one’s actually really cool:
(Minor MOON. and Kanon spoilers)
The two people on the left in the party during Mai’s route of Kanon are Haruka and Ryousuke from MOON.!
I guess you could say they’re part of the same… KANON!
So I’ve attempted to find every instance of what I call “Kerokey”, an image of Keropii holding a key. So far I’ve found these four:
This one is just obvious:
There’s one on a fan in the staff room in Clannad:
In this one I think he’s actually holding a musical note, but I’m not sure. And of course in the top right corner we have Key’s shameless self-promotion. Actually, I think it’s supposed to say “Key Sounds Label”, which might be why he’s holding a musical note rather than a key.:
And of course the one in the drama club room in the bottom left:
There are other instances where you can see what is presumably Keropii, like on Akio’s apron, an Akane CG in Rewrite that’s really obvious, the ace of spades Sakuya’s holding in a CG - if you look at it closely, and you can see Nayuki sleeping with Keropii in a CG, but I don’t count those because they’re not holding anything.
Edit: Unrelated, but I just found this One/Nanase reference in Mio’s route of LB!
LB!, where a random female student is being asked to translate a line:
Edit 2: Well, that didn’t take long…
Summer Pockets, all of it. Too many homages to count. But just to have something specific, on the last day of the trial you can help the boys as they try to take care of a beehive. They fail horribly and end up getting told of by Nomiki how kids shouldn’t try to do something so dangerous. Whatever could she be talking about…?
Not exactly Key, but close enough:
I was rereading Moon. when I noticed this background image
Which I of course recognised from One
Bit of a stretch to call it a reference with just that, but I do feel it was intentional because in that scene in Moon. (spoilers for both) Ikumi talks about going to the end of the sky and wishing time would stop forever. (Which isn’t exactly rare for Maeda, mind you, but topics like that aren’t usually touched on in Moon.)
From Rewrite, one of the optional quests
As it is stated in this post, the LOLICON brand is a thing that often appears in the earliest works of the Key staff but did you know that its first appearance was in Dousei?
It’s one of the places where the protagonist can work.
The background CGs of this game are made with real photographs. Does it mean that the LOLICON Brand was real since the beginning!? 
In AIR, it’s very hard to see normally but if you play the VITA version with the HD CGs, you can see that Michiru wears a pair of “KEY” shoes! 
The CLANNAD manga has quite a lot of references that are really well hidden.
The first one I’ve seen is a cameo of Yukito and Misuzu during the Founder Festival.
The funny thing is that Yukito also appears during the festival in the anime released later.
2 panels later, we can see a girl that looks a lot like Ikumi from MOON.
I’ve also seen some girls looking like Sayuri and a young girl at the end that resembles a lot to Misuzu but since I’m not 100% sure they’re cameos I prefer to not post them.
In the office where Tomoya works we can see a calendar where there is a picture of Piro on a cutted trunk!
Finally, at the very end of the manga, there is an exclusive scene where Tomoya participates to the school race that he wasn’t able to attend in the first timeline. Yukito is on the starting line.
(Sorry for the bad quality of the picture, the scans of the end of the manga can’t be found on the internet and I had to take a photo of my french exemplar)
I’m pretty sure the girl next to her whom you can only see the back of her head is Yui too:
Oh yeah, I didn’t see the ribbon on the top of her head!
Good job finding all of these details ! I didn’t know that we could see Yukito in the anime, I will have to be very attentive when I will watch the anime again. 
By the way, where have you found a french exemplar of Clannad’s manga ? I would like to buy one too 
Thanks! I’ve bought the tome 3 to 8 on cultura.com (they’re still available). I’ve bought the tome 2 on priceminister and I had to buy the tome 1 on leboncoin because it’s hard to find (however it was cheap and in an excellent state). But if you can’t find the first tomes, you can read the beginning of the English fan translation.
To talk a bit about the manga. The English translation stops just before the graduation of Nagisa (near the end of the tome 5), this translation was stopped years ago and will probably never continues. To my knowledge, outside of Japan, the manga was only released in China and France (since manga are very popular in our country).
I don’t recommend reading the fan TL because the manga is pretty much a slow burn and the fact that the translation stops before the best parts of the story is really frustrating, however if you can read it entirely, that’s a pretty good adaptation, covering pretty faithfully the main story with quite a lot of interesting thoughts and added stuff. I really liked the caricatural/fan service drawings and I absolutely loved the last tome, living again the end of After Story through a manga was really something.
(sorry for the short off-topic)
Thank you very much for your advices 
next time I will send you a message directly sorry 
