Unlikely Villains #1: The Mischieveous Yarn Lady
She seems harmless enough, and even talks to you about your faaaaavourite subject. She is helpful, although she also tries to sell you loads of stuff you don't need. She's in her middle ages or older, and seems quite the sweet little lady.
However, if she thinks you're not worthy, or didn't buy enough, or didn't listen to her advice, or didn't listen to her at all (didn't make small talk), she will 'forget' to pack all of your paid-for yarn into your bag, so you miss *that* one ball.