i found this response to a question on holes in yer ankles on knitters review. i'm going to paste the answer as chances are by the time i get round to my next pair the thread will have been archived or deleted.
This problem is basically very localized laddering, caused by stress on the corner when you're working the heel and the instep stitches are resting.
For covering up the ones you've already done, take a little scrap of yarn and duplicate stitch over the holes to pull them together, then weave in both ends of your yarn scrap.
For preventing them in the future, either lift the stitch from the row below on the instep side and knit it together with the first stitch on the heel side, or vice versa, or pick up the bar between the stitches, twist it, and knit it together with the next stitch. If you can still see a small gap, a little judicious work with a crochet hook, easing the slack into the surrounding stitches, will take care of it.