I do not think it is up to us to determine what is considered an Illegal Technique - TLG (The LEGO Group) determines this for their commercial products. It depends on how much of a purist you are whether you apply these constraints in your own builds. That said, LEGO Designer Jamie Berard has made a presentation in the past that highlights most Illegal Techniques. A copy is located here: http://bramlambrecht.com/tmp/jamieberard-brickstress-bf06.pdf

Some examples:

• 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 do not think it is up to us to determine what is considered an Illegal Technique - TLG (The LEGO Group) determines this for their commercial products. It depends on how much of a purist you are whether you apply these constraints in your own builds. That said, LEGO Designer Jamie Berard has made a presentation in the past that highlights most Illegal Techniques.

Some examples:

• 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 do not think it is up to us to determine what is considered an Illegal Technique - TLG (The LEGO Group) determines this for their commercial products. It depends on how much of a purist you are whether you apply these constraints in your own builds. That said, LEGO Designer Jamie Berard has made a presentation in the past that highlights most Illegal Techniques. A copy is located here: http://bramlambrecht.com/tmp/jamieberard-brickstress-bf06.pdf

Some examples:

• Studs on side bricks cannot have their side stud in the hole of a technic bricks

• 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.

I do not think it is up to us to determine what is considered an Illegal Technique - TLG (The LEGO Group) determines this for their commercial products. It depends on how much of a purist you are whether you apply these constraints in your own builds. That said, LEGO Designer Jamie Berard has made a presentation in the past that highlights most Illegal Techniques. A copy is located here: http://bramlambrecht.com/tmp/jamieberard-brickstress-bf06.pdf

Some examples:

• 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).