This is in the scene where Jenny & Matt are on their first date and end up back at Matt's apartment for there first time. If you take a look just after Jenny comes out of the bathroom and Matt is sitting on the bed, he has 2 shirts on. He has a dark, long sleeve shirt. If you look along the back of that shirt, you can clearly see the seperation where the shirt is joined together. Then after Jenny rips the shirt off, you can see a small velcro strip on the collar of Matt's grey t-shirt.