Calamity Jen - S1-E2
Continuity mistake: Near the end of the episode, Roy is helping Jen push her shoes on. While he's pushing one shoe on, you can see Jen has her shoe on her left foot. Two shots later, she is completely barefoot and the shoes are nowhere in sight. After Denholm uses the stress machine on her in her office, she walks out with both her shoes on.
Calamity Jen - S1-E2
Continuity mistake: When the stress management therapist opens the suitcase containing the "stress machine", you can see an electrode on each side of the suitcase. In the next shot, both electrodes have moved to one side.
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.