Neil Patrick Harris (Reuters/Danny Moloshok)

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A young Neil Patrick Harris enjoyed a cameo in a wonderfully self-referential sequence in season three's 'Bart The Murderer'. After Bart befriends Fat Tony's mob and is accused (and cleared) of Principal Skinner's perceived murder, Harris is seen starring as Bart in an hilariously inaccurate TV movie based on Bart's Mafia life entitled 'Blood on the Blackboard: The Bart Simpson Story', that the Simpsons watch at the end of the episode.

Harris isn't the only star to play themselves playing a Simpsons character in a spoof biopic in the show. Veteran Jewish actor Fyvush Finkel played himself playing Krusty the Clown in 'The Krusty The Clown Story' in season nine's 'Lisa's Sax', while NYPD Blue star Dennis Franz plays himself playing Homer in exploitative telemovie 'Homer S.: Portrait of an Ass-Grabber' after Homer is accused of sexual harrassment in season six's 'Homer Badman'.