Not only can the first notch, the safety notch be broken, all of the notches can be broken from careless handling and shooting practices.
Even I have occassionally had the hammer slip on the way back to full cock.
Some old Colts had all their hammer notches broken and the owners would hold the hammer back to half cock to load them.
Only a TYRO of the lowest rank would load six beans in the wheel in the old days. There is a funny story about Wyatt Earp having his peacemaker drop out of the holster and discharge and having to pay a fine.
Earp was no master gunfighter...in his biography, he claimed to hunt buffalo with a shotgun.
Seems like every four months on the internet forums somebody young rediscovers the Colt SAA or replicas and wants to know why they can't fan it, or load six rounds in it for carry ,etc...
I think a lot of this comes from western movies and books which often have such foolishness....
Remember boys, as much as it pains anybody under 30 to admit they are wrong, the reality is this gun has been around for over a century and people figured out it's quirks long before you were born.
The Colt SAA and copies have three notches on the hammer, but they make four clicks when you cock the gun. The first click is the safety notch. The second click is the half cock notch. The third click is the cylinder bolt locking into place and the fourth click is the half cock notch....
When you fan a peacemaker, in addition to putting stress on the springs and lockwork, you bugger up the cylinder locking bolt and cut an ugly circle around the cylinder.
Just load five beans in the wheel and don't fan your peice and you will be fine...