Chris Roberts, a prominent computer security expert, was aboard a United Airlines flight last week when he tweeted about a potential security flaw he found on the plane’s on-board Wi-Fi. Big mistake. Airline personnel saw the tweet and alerted authorities at Syracuse Hancock International Airport where the flight was scheduled to land. Roberts exited the plane and was quickly detained by the FBI. Roberts, the founder and chief technical officer of the Denver security firm One World Labs, said the agents questioned him, confiscated several of his electronic devices, and then let him go. “Lesson from this evening, don't mention...