Setup people who get a machine ready to run production know that just about anything can happen the first time they execute a program. If certain control panel functions are not properly set, the machine will not behave as expected.
For example, if the dry run switch is on, rapid motions will (usually) be slowed while cutting motions will be accelerated. If the feedrate override switch is not set to 100%, machining will not occur as intended. And if an offset (tool length, cutter radius, fixture, or geometry) has not been correctly measured and entered, the results could be disastrous. These are but a few of the problems a setup person can have when running programs - new programs and even programs that have been run before.
Though setup people get pretty good at handling problems, CNC operators may not possess the same skills. In most cases, a CNC operator will only take over once a setup person has made the machine ready for production.
I offer two simple and somewhat obvious suggestions. Though they're obvious, I'm surprised at how many operators don't follow them. They're guaranteed to save a crash some day.
My first recommendation is to get everyone that runs a CNC machine in the habit of having a finger ready to press the feed hold button every time they press the cycle start button. Feed hold, of course, will immediately halt axis motion as soon as it is pressed - which is just what the doctor ordered if something is wrong. But of course, this button can only be pressed once it is located. If the machine is moving (at rapid) in the wrong direction, it is unlikely that anyone can find the feed hold button in time to stop the machine before a crash - unless they're ready to press it the instant the cycle is activated.
Unfortunately, running production on a CNC machine can be rather mundane - and boring. This is especially true with long running jobs that have been properly processed and programmed. Operators can be lulled into a false sense of security if they run hundreds (or thousands) of cycles without mishap.
So my second recommendation is to get everyone in the habit of watching the first tool's motion until it begins cutting. An operator won't know if something is wrong if they don't watch the machine! Extra care should be given after an off shift, a break, lunch, or any time the operator isn't running a cycle immediately after the previous one. A control panel button or switch could have been changed during this period without their knowing it.