In the Promo artwork for Miracle Mask, you can see Stachenscarfen.
This man gets so excited about seeing the Bostonius that he rushes out his house and immediately slips on the ice:
It's not a continuity error that Dean Delmona has white hair in Laytons Mystery Journey. When he's shocked, his hair flies off his head, revealing it to be a wig.
Ingrid and Matthew have to stand on piles of books to be able to see out the window:
Duke makes an appearance in St Mystere despite being from Folsense. And being a hallucination.
The man Grosky fights with in Last Specter can be seen in Prison in the credits of Eternal Diva.
In Eternal Diva, during Puzzle 001, you can overhear two men arguing about the answer.
The two are looking at a fossil of what appears to be a Ichthyosaur.
You can hear the first man ask, "What about that? Is that old?"
Second man replies, "That's a fish."
The first argues back, "It's a fossil you buffoon!!"
I don't know, I thought it was funny.
That's all I can fit onto this post.
Please share any small details you've noticed, I love seeing them!