Yes - and it went horribly wrong.
Driving home from work a few years back, I saw a teenage girl jump out of a car in front of me. The driver stopped his car and ran after her, grabbed her whilst she screamed and tried to struggle free.
I jumped out of my car, grabbed him by the neck, and pulled his arm behind his back, whilst asking the girl if she was alright.
It was at that point that it all went pear-shaped. He was a cab driver, and she had robbed HIM. She legged it as soon as I had the driver off her. She'd even cut him with a razor.
I couldn't have felt worse. For all the right reasons, I intervened, but effectively - I aided and abetted her getaway.
Would still intervene though - got into a massive fight with a bunch of homophobic thugs who were picking on a gay chap sitting on his own on a night bus. That was very silly as the odds were seriously stacks against me - but if I'd have read in the papers the next day, that the chap in question had been beaten up, or killed - I wouldn't have been able to live with myself.
Hmmm...could it be argued that on some level, bravery is an act of selfishness, rather than selflessness???