I was at a quilting retreat last fall witnessed a rotary cutter accident for the first time. Those blades are really sharp and go through tissues very quickly. Luckily, there were several retired nurses at the retreat who were prepared for the accident. What this experience taught me was that every sewing kit should include gauze, first aid tape, and finger cots.
I once dropped the rotary blade (not in the cutter) while changing it, and it fell into my lap. It bounced off one leg, then the other, and then landed on the floor about five feet away from me. It caused a matching pair of small cuts (one on each leg) that stung but barely bled. Now I am sure to change the blade over a table and not over my lap.
I've read of four or five incidences of open rotary cutters falling off the table and landing on someone's foot. Every account ended with a trip the ER for stitches. I have a couple of dogs that are often underfoot when I am sewing, so I created the habit of closing the rotary cutter at the end of every cut. When I work with other quilters, it seems that I'm constantly closing rotary cutters set aside by someone else.