Very shortly after the production of Every Man in His Humour Jonson became involved in an argument with an actor and erstwhile colleague from the Admiral's Men, Gabriel Spencer. The quarrel may have arisen from Jonson's recent defection to the Lord Chamberlain's Men, or it may have been entirely personal. Whatever the case a duel was fought in the fields of Shoreditch, close to the Theatre, and Spencer was killed by Jonson's sword. The playwright only saved himself from the gallows by pleading benefit of clergy—that is, by proving he was literate and could read. His thumb was branded with the letter "T," for Tyburn, so that he would not escape a second conviction.
In case you don't know: The Lord Chamberlain's Men were, at the time, the troupe that Shakespeare wrote for and acted in, rivals to the Admiral's Men. The Theatre referred to here is a specific theatre named that, which was responsible for bringing the Latin term, theatrum, into regular—and eventually generic—use as a term for a place where plays are produced. The murderer's brand refers to Tyburn because that was the site of the London gallows, at the present-day site of Marble Arch. And though the value of pleading benefit of clergy diminished over the century or so after Jonson's plea, the option wasn't formally abolished until 1827.
Then in 1600, Jonson gets himself in trouble again, this time over a specific literary quarrel.
In his next play, The Poetaster, he ridiculed [playwright John] Marston as a hack poet and plagiarist. Marston eventually counterattacked with What You Will, in which Jonson was lampooned as an arrogant and insolent failure. In his aggressive manner Jonson then challenged Marston to a duel; since he was already branded on the thumb for murder, this was a foolhardy strategy. He probably guessed, however, that Marston would decline the challenge. Jonson then sought his man in the taverns of London, and found him. Marston pulled a pistol, whereupon Jonson took it from him and thrashed him with it. That is the story that went around London. Jonson repeated it later.
And Peter Ackroyd repeats it now, which is why he is always worth reading.
Somehow, the young hothead Jonson lived to die of old age in 1637 at 65, whereupon he was buried in Westminster Abbey under the inscription, "Oh, Rare Ben Jonson."