One of my favorite building techniques is to place a Technic pin into the anti-studs of a brick. This allows two bricks to attach bottom-to-bottom. It's a pretty secure connection, but I'm not sure it was intended to be used this way. Has this building technique ever been used in an official LEGO set?
I'm not aware of this being used in any official sets, but I'm happy to be proven wrong. I can say that LEGO considers this to be an "illegal" connection. The Technic pins need room to spring back out and be "in click", or they can be permanently damaged by prolonged assembly. LEGO would not allow this connection to be used in a current set.
A Technic pin not "in click" was used back in 2002 in the Audi TT and led to part damage as well as this sort of connection being banned from sets:
No, LEGO did not intend to do that building method though I have to admit, It is a pretty cool technique. So far there haven’t been any sets like that but maybe in the future LEGO people will realize they can do that.