So I carefully position the metal straight-edge over the balsa sheet, lining up with the marks, so that any cut that goes wide to the right (I am right-handed) will be on the 'waste' side of the material. I carefully position the X-acto (sharp, of course, so it will cut smoothly) next to the metal straight edge, with the cutting edge lowered close to the balsa, and the back edge of the blade held up vertically. I know that if I hold the blade in this position then the cutting edge of the X-acto will not bite into the metal straight-edge and leave the balsa. So I am drawing the X-acto slowly and carefully along the straight-edge, and of course the blade tips just enough for the X-acto to grab onto the metal and start riding across the top of it. It is headed for my fingers, which are also on top of the straight-edge to hold it down securely on the balsa.
Self to self: "Hey, the blade jumped up on top of the straight-edge again. We should stop cutting now before we reach our fingers."
Self back to self: "You know, you're absolutely right, we SHOULD stop cutting now. It's getting awful close."
Self to self: "Well, when are you gonna stop? Why are you still cutting? Don't you remember what happened last time?"
Self again: "Of course I remember last time, that was a really really stupid thing to do, why would I want to forg . . . "