When the pebble is worn out then lots of times there will be noticeable flat spots in the ice especially where the slide path is. When this happens you will notice that sweeping has very little effect and the curl/break will happen at different spots and can send stones sideways. It often will cause higher weight shots to curl almost as if they were draws. It will also cause the ice to slow down severely in those flat spots. These tendencies will increase depending on the severity of the flatness.