Coming down 12 O'Clock Knob in Salem, VA I have had the brakes go from a nice abrasive state to an adhesive one (i.e. squealing). Rubber pads are not meant to operate this way and the result is decreased braking power. It didn't cause me any real issues but it definitely stank and I had to sand off the glaze afterward. That was on Ksyrium SL wheels with Shimano Ultegra brakes from the 9 speed era.