I think I have, but I don't know how long the bloke would have been beaten... Someone I vaguely knew was having his skull smashed in with a large bike D lock, by an even vaguer acquaintance. It was messy. You could see his skull crunching in. It started from nowhere, attacker was on crack and it was a miscommunication over a woman.
Everyone was just watching in shock. Loads of people there, all in broad daylight, city centre. It was like I was frozen, then I saw/heard his skull go and walked in between them. Grabbed the lock, managed to snap him out of his crack zone (Probably foolishly, but i had dealt with many cracked up twats by then). Held this bloke, who's name I have never known, until the ambulance turned up, while he bled all over me. I was blood soaked from neck to foot.
Then I walked the 5 minutes home, put my entirely white outfit on to soak, got changed and went straight back out. Never mentioned it (after that day) again. Never told my family. First time I've ever really explained it actually! (Not the sort of situation where police were involved.)
I got stopped and asked if I needed help by 5 big men on the 5 minutes back (covered in blood, remember). I was the only person, out of literally hundreds watching, to do anything while it was happening though... I was 16 and tiny at the time. Has always stuck with me, the realisation that you could be beaten to death in public and have no one to help, if you were the wrong sort of victim.