A die block cracks in the furnace. Three weeks of machining, gone. The certificate said D2, the bar looked like D2, and the invoice for the Die and Tool Steel was paid in full before anyone ran a hardness test.

A 90 mm bar goes onto the lathe, and three passes in the insert chatters on a hard spot. Or the finished shaft measures oval by half a millimetre and nobody can say why. Here's the usual cause: the order said grade and diameter, and nothing else.