This is a big question for me. What did Sherlock understand when he finally saw the ring?

It almost makes me think that he demonstrates to John how easy it is to get engaged, how it doesn't necessarily mean what it might be taken to mean on the surface. I know that the engagement to an employee of the blackmailer was part of canon, but it could also serve an interesting purpose in this adaptation. As they are closed into the elevator, Sherlock repeats the line about "human error".