I very nearly got into serious trouble after trying to help a homeless person out. There was a homeless guy playing an acoustic guitar and singing and he was very very good. I've seen enough people tanked on booze and drugs to know when someone is either high or drunk, and he looked pretty presentable considering the clothes he was in and the position he was in. So anyway, as I was on my way back to work on my lunch break, I had a couple packs of sandwhiches from boots and I gave him one. He was clearly very greatful and I was happy to have helped. Anyway, about 15 feet later there was another guy that looked at me, not hopefully, I don't really know the look, but he looked at me in a weird way and I thought something was up straight away. Well anyway I carried on walking and I could smell the booze as I passed him. He shouted to me asking why I wouldn't give him anything, and then I felt a hand in my pocket... I put my hand down and grabbed his wrist and pressed my fingers in against his veins until he let go and then he started to push me. Well anyway, he went to push me in the chest and I put my arm up and his arm glanced off into my throat and it winded me a bit, strange feeling having pressure there I guess. So I hit the guy and he ended up with a broken jaw and fractured cheek bone. I kinda felt bad afterwards, but I was infuriated by the fact that I was being told I might be in the wrong, when I was only defending myself.
So now I steer well clear.
That's my experience and I know others have had bad experiences, but I also know some people have had good experiences. But you should always be wary when you extend a hand, it can be more trouble than it's worth.