Original: Littleton became widely known in 1999 when the Columbine High School massacre occurred at nearby Columbine High School, which some in the news media erroneously reported as being located in the city.
Simplified: It is most famous in 1999 because of the Columbine High School massacre at Columbine High School, which is not actually in the city of Littleton, but is in a ZIP code assigned to Littleton.