Yes I wish I had taken a photo, it has happened 2 times and then I waited to hear any answers so have not tried again. If it happens again I will post.
It's not the slack at all... the yarn is live, that is, it is coming from a live skein. As such there is unlimited yarn... it is just sticking out the right side instead of staying inside. I know it's terribly hard to imagine without a pic!
If anything I am knitting too tight, since the ankle part has drawn in and I don't know if the baby's foot will fit through!