I was running a repeat job for some simple spacers out of plastic. Somehow, I entered the wrong Z-axis depth by a whole inch. The drill plunged right through the part and into the fixture plate below. It made this HORRIBLE grinding noise that scared everyone in the shop. We ended up with a spacer that had a surprise hole straight through the middle. The boss just shook his head and called it a 'design feature'. Now I ALWAYS verify my offsets twice before hitting start.
Started doing full warm-ups and my machine runs quiet now.