Continuity mistake: Roy closes his comic and throws it onto the desk. The camera flits back to Moss, then back to Roy, and Roy is holding the open comic again and repeats the same actions. (00:06:25)
Continuity mistake: Roy has scratches on his face from his and Moss' 'fight'. In the next scene, the scratches are gone. (00:15:30)
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.