Freemasons have an assortment of secrets, including the fact that they exclude atheists, their symbol is printed on the dollar bill, Anders Breivik was one of them, and they had an active role in the space race. These are secrets that have been exposed to the public.
One secret of Masons is that they actively deny membership to atheists and agnostics. While they have no specific religious affiliation, it is a secret requirement to believe in some kind of a higher power to gain membership. That being said, Masons do not exclude traditionally ostracized groups such as homosexuals.
Additionally, a previously hidden secret of Masonry is that their symbol is printed on the U.S. dollar. The All-Seeing Eye is printed at the apex of the pyramid on the dollar, along with the Freemason motto "Annuit Coeptis," which loosely translates to "new world order." It is said that this printing is pure coincidence and not the brainchild of a specific Mason.
Additionally, Andres Breivik was a Mason. He was responsible for the 2011 mass murder in Norway. His level of involvement is unknown, and he was promptly removed from the organization following his murder trial. Finally, several of the American astronauts involved in the Apollo program were Freemasons, Buzz Aldrin included. Some people refute the legitimacy of these secrets, but they have mostly been backed up with evidence.