i've had this happen on machined brake surfaced rims only. sun rims and kinlin rims. otherwise, the rims are fine. it looks to me like the machining is too coarse and leaves furrows on the surface. after some use the high areas slough off and can embed themselves in the brake pads. usually it is temporary, and a good hard stop or two will dislodge them, but on occasion i have had to remove the brake pads and dig them out with the point of a knife. this is not the end of the world...