Living in Seattle, there are certain parts of downtown that I would normally feel comfortable walking through but was warned by locals about gun and drug violence.
There are many examples of lost tourists in Mexico or South America that get approached by drug traffickers on YouTube.
It is extremely difficult to go wrong in Vietnam, but there are many places that appear peaceful but are actually run by crime.
Of course when I visited her home town and walked up a hill for the view one afternoon, I didn't think that much about a syringes off the side of the trail, but her family lost their minds that I had been in the most dangerous part of the city, "people are killed there all the time".
We are terrible about judging dangerous areas outside of our cultural experience. Many westerner's can't even identify a RedLight area in Asia, "Why are all those women wearing such short dresses?", "all those girls (actually boys) are really dressed up nice".