It is usually nothing to be concerned about. If this happens often and you're trimming every 8 weeks, you may consider bringing it in to 6 weeks. My experience has been that it depends on the horse, the time of year, and how much riding you do (and the terrain)...they often wear unevenly. If you want, when your farrier comes out, ask him to show you how to rasp them down to clean them up. Years ago, our old farrier convinced me that I could do this myself and not need him anymore. After a few times having him out to check my work and give advice, I've been on my own for a couple years and have never had a problem. It's not hard...I do one of our 3 mares a weekend, so they all get done every 3 weeks using only a rasp.