Continuity mistake: Near the start when Moss runs and jumps onto the couch, his tie changes positions repeatedly between shots. In close-ups it is up on the couch arm and in all other shots it is under Moss.
Continuity mistake: When Laura Knightly leans against Roy for the cameraman to take a couple photos, her face is even with Roy's. In the next shot she's above him so her chin is at Roy's eyes.
Continuity mistake: Season 2, episode 5 "Smoke and Mirrors": Roy's lipstick alternates between bright pink and more of a peachier colour throughout the scenes.
Continuity mistake: When Jerome pulls one of the disabled men out of his wheelchair, both of the disabled men on screen are in different wheelchairs to those they used in the scene where we first see them (when Roy files the police report).
Answer: The shirt is available from www.ThinkGeek.com and is fairly popular in the nerd culture, so it's possible you've seen it in real life or online.