What is an illegal building technique?
An illegal building technique is a Lego piece connection that is officially listed as illegal by the Lego corporation. They are never used in official Lego sets. This is because these building techniques stress the Lego pieces beyond their intended use and cause the pieces to get damaged over time.

For example, sandwiching a Lego plate in between two Lego studs is an illegal technique as this was not their intended use, and the distance between the Lego studs is not wide enough to fit the Lego plate without damaging one of the pieces.
Instruction: What is an illegal building technique?
Context: 
Response: An illegal building technique is a Lego piece connection that is officially listed as illegal by the Lego corporation. They are never used in official Lego sets. This is because these building techniques stress the Lego pieces beyond their intended use and cause the pieces to get damaged over time.

For example, sandwiching a Lego plate in between two Lego studs is an illegal technique as this was not their intended use, and the distance between the Lego studs is not wide enough to fit the Lego plate without damaging one of the pieces.