Most machines use bearings. Failed bearings are often particular items of interest regarding the evaluation of a failed mechanical device.  However, it is usually difficult to determine whether a bearing caused the failure or failed as a result of the rest of the machine failing (overload, contamination, overheating, misalignment, etc...)
An interesting scenario that often arises is when the bearing is the only failed item, and upon our arrival the machine is perfectly filled with brand new oil or grease.