The machine plunged into a large block of aluminum at full rapid speed with a 1 inch rougher end-mill. It then proceeded to run the cutting program, again, at full rapid rate. The sound it made is something that I will never forget. This thing was churning aluminum like it was butter. It was literally turning it into goo, not cutting it at all. It did not care one bit. The cutter was large enough and stubby enough that it did not break.
These things are fast enough that, by the time your shocked brain is able to respond it could have killed and dismembered you five times before you can even think of slamming the e-stop button.
The end-mill came out with a ball of aluminum, 2 inches in diameter, permanently welded to it. It was one of the scariest industrial accidents I've been around. Thankfully the enclosure and rigidity of the machine made it so that the only consequence was aluminum that looked like churned butter.
Years later I worked with an aerospace client who uses stir welding to assemble rockets. Same process, except you are doing it on purpose and with controls. The idea is that stir welding preserves the metallurgical properties of the material, whereas TIG/MIG welding might not preserve properties important to the design.
To echo your other comment, since that happened I learned to never trust CAM software or simulations, always run it on air first and, if necessary, machinable wax blocks. Decades later, I still don't consider myself a machinist at all. I am pretty decent at it. It's a complex art that requires time and dedication. I do too many other things to be that guy. I do have a VF-2 and a Bridgeport knee mill (full manual still) in my garage though...
I have a 30 Watt diode laser that came without any safety measures at all. It kind of surprised me that you could just fire it with a G-code and absolutely nothing stood between you and the beam. It cuts wood like butter though, up to 20 mm in a single pass (solid wood, not birch ply) and I'm super happy with it but that tool is going to go through some serious modifications to make it safe. It can set just about anything on fire fair distance away and will fry your eyeballs from half a kilometer if not more. Any idiot with a few bucks to spare can go and order one and they have absolutely no clue about the kind of damage a machine like that can do. It looks so innocent, it's completely deceptive.