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.
Revealing mistake: When Moss runs into the door, all walls on the set shake. (00:06:25)
Friendface - S3-E5
Other mistake: In the beginning of the episode, Roy is explaining why Friendface is bad and chastising Jen for being on it. If you look at his monitor, he is on a website with a green color palette like the Friendface infomercial and a layout similar to MySpace. While I can't be 100% sure, it seems like a graphic designer created a mock website of Friendface to be used in the show. If true, that means Roy is already on Friendface before Jen even enters the room. (00:01:00)
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.