Very common. If you take a look at some of the posts already here on the forum, you'll see that. Happens sooner to some folks, later to others. Other than being somewhat unsightly, there appears to be very little difference in comfort, and none in utility. Hopefully version 2 will address this, but in the meantime its either live with it or try for a replacement (usually not a huge problem, I gather. Haven't bothered to replace mine).