I had a young sewing student last year whom I warned about using a rotary cutter: "this is the most dangerous tool you'll ever use sewing. You can permanently injure yourself with this....". I then demonstrated the proper use of the rotary cutter: "Make sure your hand is on your ruler like this....look carefully again before you start cutting....always cut only away from yourself...". I hovered right next to him as he made a few careful and successful cuts, and then from about three feet away I watched him position his hand so the cutter was going to run right over his thumb for sure. Luckily I was able to make a grab of his ruler hand just in time. We stuck to scissors after that.