Mainly due to military FMJ use, some rounds are designed to tumble, so that may not be technically a failure, if that's the design. So, I'd say it fails if it does not perform as designed. That's pretty much along the lines of Whit's above post. The confusing part is that a bullet failure can result in an ideal game kill. Sometimes an autopsy is required. It has been an issue for a long time. Al Georg placed a brass screw in the nose of a hollow point bullet and killed a bear with it. Later, folks milled slugs from architectural bronze, and other metal, to increase performance. It is a fascinating subject.