I am always wary of buying items off the street from vendors in general. After all, the food is not closely regulated and who's to say that someone selling kabobs is actually offering a meat that's not say, rat?
In the news today, there was an article about Puffer fish being painted pink to resemble salmon and sold to unsuspecting consumers. Your basic puffer fish has a poison called "tetrodotoxin"in its ovary, liver and intestine. If any of this poison gets in the meat or is consumed by the eater, it can and most likely will kill.
I'm not saying that every street vendor is unethical. In reality, a high percentage are most likely above board. Just be extra cautious. Especially if you're nibbling in the market in Thailand.