yep, i was using my 24 inch ruler that has a plastic guard along one side. but of course i'd picked it up and was cutting on the side without the guard.
i evidently hit something and the cutter went up over the ruler and onto my thumb. cut it badly. bled for 4 hours and went to ER, sat another hour and it stopped by then. all they did was wrap miles of gause around it. later i bought a metal finger guard since i'd cut nerves that were very sensitive for weeks afterwards.
i'm much more careful now!! also use a cutter that has the handle you squeeze to open the blade and when you let go the safety shuts it automatically. good in case i were to drop it.... ouch!