Another way to fudge that unexpected hole that won't go away... but won't get worse: take a length of the same yarn, thread it across your hole so that half the hole is on either side of your yarn. Now simply tie a knot in your length of yarn, closing up the hole.
Take your crochet hook and work/weave in the ends like you would if you'd had to tie on a new ball of yarn.
Hopefully this little fudge won't be noticeable by anyone but you. I know I've had to use it more than a few times.