I once tried building a Macbook app which lowers my music volume when I'm not smiling, to try to stimulate more happy emotions (forcing yourself to smile induces happy feelings)

Then I started thinking how this could be used in restaurants to see if waiters smile to the people they are serving Or in customer service (you can actually hear it when people smile on the phone)

Then I realised that this kind of tech would definitely lead to abuse

(btw that's not the reason I didn't build it, it was just not that easy to build)