The only time I've ever known of a discharge when a safety was clicked off was on a Savage M340. I owned the gun, never modified anything, not sure the trigger was even adjustable as it was a cheap gun to begin with. Sold it to my Uncle and it happened to him about 5 years later. No one was hurt, thankfully. He sold the gun. Not sure if he sold it broken, or fixed it.
I KNOW that if an adjustable trigger is not properly adjusted, this can happen. This is why I had a good gunsmith set my Remmies, didn't attempt it myself. Everything is inclosed and I can't tell what's what with it, so figured it was money well spent. Didn't have the net back then or I could have googled a tutorial, but I'd still feel better about having a qualified gunsmith that's done it hundreds of times, take care of it.