High end CNC controls have chuck barrier software built in that makes this less likely. It needs to be skillfully set up. High end shops may also have good 3D rendering software that can be used to prove out toolpaths. The programmer will model his machines and fixtures accurately so he can then prevent crashes. Average CNC controls at least have line output graphics that prevent non-subtle mistakes. Finally, the operator will run the first part peering into the machine, with non-cutting rapid traverse moves artificially slowed down, and with her finger poised over the feed hold button.