This most recently happened with my trapper pistol but since set triggers are mostly a rifle thing I am posting here. I had a similar incident with my TC hawken a decade or so ago. I didn't have an AD with the hawken but it was close.
I was teaching my lady friend Isa how to shoot my trapper pistol last winter and she had a very scary AD when she went to set the trigger and the hammer fell from half cock on to the nipple and fired. Apon inspection I determined that the trigger adjustment screws, both of them had loosened up from recoil. I had put about 500 rounds through the new pistol. I took the lock apart, inspected it. read the owners manual and properly adjusted both triggers. two different screws. Once I had it correct I then super glued both those set screws in place. No problems since with perhaps a thousand rounds fired post fix. I now periodically check the trigger by firmly pulling the trigger when its on half cock and empty. if the hammer drops from a trigger pull on half cock the trigger is not adjusted properly and the gun is very unsafe.
I was teaching my lady friend Isa how to shoot my trapper pistol last winter and she had a very scary AD when she went to set the trigger and the hammer fell from half cock on to the nipple and fired. Apon inspection I determined that the trigger adjustment screws, both of them had loosened up from recoil. I had put about 500 rounds through the new pistol. I took the lock apart, inspected it. read the owners manual and properly adjusted both triggers. two different screws. Once I had it correct I then super glued both those set screws in place. No problems since with perhaps a thousand rounds fired post fix. I now periodically check the trigger by firmly pulling the trigger when its on half cock and empty. if the hammer drops from a trigger pull on half cock the trigger is not adjusted properly and the gun is very unsafe.