The good folks at DMM have put together a nice video on this topic. Check it out, below:
A very similar issue takes place when draws are fixed permanently on sport routes. Draws are generally only affixed to routes that are very hard where the climbers working the routes tend to take a lot of falls. Eventually, the rope ends of the carabiners begin to get worn down by the ropes running through them and can create sharp edges.
The take-away here is not just to pay attention to where you are using your carabiners, but also to constantly check your gear. Look for damage on the carabiners that could lead to rope damage. And look for damage in the rope that could lead to failure...
Jason D. Martin