The following may or may not be true, but it is amusing that in Mexico, where gun control is very strict (except for criminals, of course) this supposedly happened.
"In a country in which the government cannot or will not enforce the laws and protect citizens?the United States comes to mind?said citizens will want to protect themselves. This is happening in Mexico. In a recent example in Mexico City (this from newspaper accounts) four robbers, armed, boarded a public bus and collected wallets and cell phones from the passengers. One passenger drew a pistol and shot all four, killing one. The other three, badly wounded, got off the bus. The passenger followed, finished them off, gave the loot back to the other passengers, and disappeared into the city. Curiously, not one of the passengers was able to describe him. Maybe they were not paying attention."
--- Fred on Everything