One common trait that is often present with autism is the lack of fear of danger, or a lack of awareness to apparent danger. With children, and even some adults this might look like a person jumping into deep water even though they can't swim, or jumping off of a high place without regards to bodily harm. Some individuals might step out into busy roads without looking for oncoming traffic. I think those are all obvious actions that stand out in a big way.
But, some may ask; Is there less obvious situations where an autistic person might find themselves in an unsafe situation, and not know it? The answer is unfortunately, yes. The older we get the more sophisticated the situations become. Some of us might find ourselves elbow deep in social situations that seem straightforward to others, but elude us.