Caught the headlines recently about Twitter's new system for blocking tweets on a per-county basis and South Korea's indictment of an activist for reposting messages from the North Korean government's Twitter account? It makes us wonder: Just what are the red lines in the United States for using the microblogging service? After all, we know that the Department of Homeland Security (DHS), FBI, and Air Force are all already monitoring Twitter or seeking to develop technology to do so. While the legal terrain here is murky, here are five behaviors you might want to avoid:

• Threatening violence: Earlier this week, DHS agents detained Irish traveler Leigh Van Bryan and a friend at Los Angeles International Airport and sent them back to Europe after Bryan tweeted that he was going to "destroy America" and dig up Marilyn Monroe during his trip -- references, he later told officials, to partying and the comedy show Family Guy, respectively (the incident conjured up memories of other jokes gone awry, such as when the Onion enraged the U.S. Capitol Police by tweeting, "BREAKING: Witnesses reporting screams and gunfire heard inside Capitol building"). In 2009, FBI agents arrested an Oklahoma City man named Daniel Knight Hayden for threatening on Twitter to kill police officers during a Tea Party tax protest. Hayden was sentenced to eight months in prison.

• Coordinating unlawful behavior: Pittsburgh police arrested a New Yorker named Elliot Madison for using Twitter to alert anti-capitalist protesters about police movements during a 2009 G-20 summit. The criminal complaint claimed that Madison had helped demonstrators engaged in unlawful behavior avoid arrest, though the charges were later dropped.

• Pranking police: This past summer, the L.A. Sheriff's Department opened a criminal investigation after the rapper The Game urged his 580,000 followers to call a number -- the department's Compton station, to be precise -- if they wanted an internship with him. The hundreds of calls that followed "overwhelmed the emergency phone system and delayed emergency service," according to the Los Angeles Times (the department ultimately decided not to pursue charges). Twitters accounts impersonating police departments in Virginia and Texas have also been shut down.

• Cyberbullying: Last week, police in northwest Arkansas arrested three girls and one boy who were allegedly sending vulgar and derogatory messages from Twitter accounts.

• Not tweeting: The manager for teen pop star Justin Bieber and an Island Def Jam Records executive were arrested a couple of years ago for not immediately cancelling an appearance by Bieber at a Long Island mall over Twitter, as police requested afters fans grew unruly. Prosecutors charged the manager, "Scooter" Braun, with reckless endangerment and criminal nuisance but later dropped the charges in exchange for Bieber recording a public service announcement on cyberbullying.