I am defining illegal as it is in this answer, specifically the following
- Studs-on-side bricks cannot have their side stud in the hole of a technic brick
- Bricks mounted SNOT style onto Ehrling/Headlight bricks cannot sit directly above a normal stud (the minimal height of the LEGO embossing makes that combination illegal)
- Technic pins have to be fully inserted into technic holes, otherwise they stay in compressed state which causes stress on the element.
- Plates cannot be inserted with their studs into technic bricks, unless it is a 1x1 plate (so only 1 hole is used) and there is no brick attached to the technic brick directly above the inserted plate.
- As mentioned by you already, you cannot insert plates between studs (this used to be a valid technique 20+ years ago, but is no longer considered "legal" by LEGO).
I am also including one other situation:
A round piece between four studs as stated in a comment by Sandra.
When, if ever, have any of these methods been used in an official Lego Build?