The line between youthful rebelliousness and something more dangerous is not always clear. But in her angry musings on Twitter, Alba González Camacho, 21, who describes herself as a “very normal girl,” sailed across it. After she posted messages calling for a far-left terrorist organization to return to arms and kill politicians, Spain’s national court convicted her of inciting terrorism using a social media network. It was the first verdict of its kind involving Twitter posts in Spain, and the case has touched on issues of where precisely the cultural, political and legal red lines lie in a country that not long ago lived under both the grip of Fascist dictatorship and the threat of leftist terrorism. The case is also one of a recent handful that have pushed social media into courtrooms worldwide and raised issues of the limits of speech in the ether of the Internet.