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: 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: Season 3 Episode 1 'From Hell' When Roy says "Nice glasses" and Moss replies with "Not as nice as your momma's glasses", The Red Door between Roy changes from being closed to being open.
Continuity mistake: The way Jen's books are stacked changes after she says "pedal stool." The book with the red pages moves to the top.
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.