I have had one such experience, but I had no gun, and was only 20 at the time, so could not have had a permit in my state anyway.
But I ran to what I thought was a mugging in progress, on a downtown street.
It actually turned out to be a domestic assault between a homeless husband and wife.
Being in the middle of that was not fun... but if I saw a guy knocking a woman to the ground and kicking her like that again, I'd be right back in the middle.
It ain't a good feeling when the voice behind you says, "Don't hurt my husband!" Luckily, I didn't get stabbed.
She opted to leave with him, too.