Stressing the elements
Official LEGO set designers has rules external builders doesn't have to adhere to. These forbidden connections are called "illegal". By not using illegal connections you can avoid LEGO bricks getting stressed and, eventually, getting deformed or snap. This presentation by Jamie Berard was held at a Brickcon in 2006 and provides a few examples of connections that are legal and their illegal counterparts. While this is a great reference when it comes to understanding the concept of legal and illegal connections, please keep in mind that this is only a glimpse into the quality assurance at LEGO. Some of the illegal techniques in this document are now legal. We at Swooshable want you to experiment and play first and foremost.