My two front fog lights were cracked within a time period of about two weeks after I have the car for about two years. I am 98% sure that the crack was not caused by small rocks and I barely use the fog light. It seems that the crack was broken from inside and started from the inner corner. BMW replace two fog light without any charge. I am happy about that
, but still wonder what causes the crack. I have the pics but dont know how to post it here.