When Billy fakes his own death and is stabbed in the bedroom, it is clear that there are no rips in his T-shirt at all. Yet later on, when Billy is in the kitchen, there are rips in the shirt.
Maybe you just couldn't see them when he turned around because he did have his hand over where he was stabbed. and his arm was blocking a lot. Plus was this before or after Stu and Billy were stabbing each other?